Go to Project Planning Board. Click on the "Options" icon. 
- Select the "Scheduling" option. 
- On the tab "Scheduling parameters for activities", the "Scheduling method" selected should be "X Network determines dates". In your case I believe it is "WBS determines dates".

You can set this in SPRO-->Project System-->Dates-->Date Planning in WBS-->Define parameters for WBS scheduling--> Select Scheduing method as "X Network determines dates"

T-Codes List Used In SAP-PS For Customization Part 3

OPN0    Master Recipe Profile
OPN1    Maintain Profiles for Master Recipes
OPN2    Valuation Variants - Prod. Order
OPN3    Overv.Var.: Master Recipe Scheduling
OPN5    Field Seln: Std Network Overview
OPN6    Field Selection Standard Network Hdr
OPN7    Field Sel.:Std Ntwk Activity Detail
OPN8    Field Sel.:Std.Ntwk Comp.Overview
OPN9    Field Sel. Std Ntwk Comp.Detail
OPO1    Create Cost Element Planning Layout
OPO2    Change Cost Element Planning Layout
OPO3    Display Cost Element Planning Layout
OPO4    Create Stat. KF Planning Layout
OPO5    Change Stat. KF Planning Layout
OPO6    Display Stat. KF Planning Layout
OPO7    View Maintenance V_TCJ41_7
OPO8    View Maintenance V_TCJ41_8
OPO9    View Maintenance V_TCJ41_9
OPOA    View Maintenance V_T003O_N0
OPOB    View Maintenance V_T003O_N1
OPOC    View Maintenance V_T399X_N0
OPOD    View Maintenance V_T399X_N1
OPOE    View Maintenance T_T399X_N2
OPP1     Customizing MRP
OPP2     MPS
OPP3     Customizing Repetitive Manufacturing
OPP5     List Profile Component List
OPPA     PP/MRP Customizing Explosion
OPPB     Direct Procurement
OPPC     Period Grouping
OPPD    Customizing Planning Time Fence
OPPE     Conversion Plnnd Order -> Prod.Order
OPPE01  General iPPE Customizing
OPPE02  Define iPPE Node Type
OPPE03  Define iPPE Variant Types
OPPE04  Define iPPE Alternative Types
OPPE05  Define iPPE Relationship Types
OPPE06  Customer-Spec. Model Assgts (iPPE)
OPPE07  Time Analysis: Partner Products iPPE
OPPE11  Profiles: iPPE WB Professional
OPPE12  Tabs: iPPE Workbench Professional
OPPE13  User Assgmt: iPPE WB Professional
OPPE14  Define Reports for iPPE WB Prof.
OPPE15  Define Interface for iPPE Workbench
OPPE20  Profile Maintenance iPPE PS
OPPEACT01       Define Std Val. Determin. Type
OPPEACT02       Object Dependency in Process Struct.
OPPECHK01      Customizing: Consistency Check
OPPEDOK          Documentation
OPPELUI01        Profile Definition: iPPE WB Express
OPPELUI02        User Assignment: iPPE WB Express
OPPERES01       Customizing for Production Resources
OPPESCMPV      Object Dependent Status Management
OPPESTATUS    Cross-Application Status Management
OPPF     Customizing for Order Report
OPPH    Customizing Purchase Req. Conversion
OPPI     Available Stock
OPPJ     MRP Checking Rule
OPPK    Hierarchy Element
OPPL     MRP
OPPM    Evaluation Profiles
OPPN    Layout
OPPO    Hierarchy Element
OPPP    Customizing Direct Procurement
OPPQ    C M MRP Plant Parameters for MRP
OPPR    C M MRP MRP Group
OPPS     Strategy
OPPT    Strategy Group
OPPU    Strategy Group for MRP Group
OPPZ    MRP Group
OPR1    Area of Responsibility <-> Message
OPR3    Definition of Breakpoints
OPR4_ACT            Multilevel Actual Settlement
OPR4_CK Material Cost Estimate
OPR4_CKMC         Mass Costing - Sales Documents
OPR4_CKML         Closing and Calc. of Periodic Price
OPR4_CKPF          Price Update
OPR4_FCO            Collective Processing: Variances
OPR4_KKA           WIP Calculation
OPR4_KKP           Repetitive Mfg and Process Mfg
OPR4_KKS           Collective Processing: Variances
OPR4_KKS1         Individual Processing: Variances
OPR4_PPCO         Production Order: Cost Calculation
OPR5    Definition of Error Mgmt IDs (SAP)
OPR6    Definition of Object IDs (SAP)
OPR7    Def. of Areas of Responsibility
OPR8    Def. of Minimum Message Types (SAP)
OPR9    Def. of Reference Objects (SAP)
OPRCMFE User-Defined Messages
OPRF    Maintain Quotation Prefixes
OPS      Customizing for Project System
OPS0    Maintain print control for networks
OPS1    Maintain User Fields
OPS2    Maintain Relationship Texts
OPS3    Maintain PS Text Types
OPS4    Maintain simulation profile
OPS5    Maintain Standard Network Profile
OPS6    Maintain Project Manager
OPS7    Maintain Applicant
OPS8    Materialflow network
OPS9    Budget Management Profile
OPSA    Maintain Project Profile
OPSB    Cost Planning Profile
OPSC    Create network types
OPSCAS  Command File Payments (PS)
OPSCOS  Command File Costs (PS)
OPSD    Maintain Profile for Report Lines
OPSE    Maintain Report Groups
OPSF     Maintain Report Variations
OPSG    Order change management profile
OPSH    Maintain ctrl. key for activity (PS)
OPSI      Edit PS validation rules
OPSINS  Command File Info System (PS-F)
OPSJ     Maintain Project Coding
OPSK    Maintain Special Characters
OPSL    Project Info System Overview Maint.
OPSM   Maintain Overall Profile PS InfoSys
OPSN    Edit PS substitution rules
OPSO    Maintain Project Types
OPSP    Capacity availability check
OPSPAR1 Maintain Partner Functions
OPSPAR2 Define Lang.-Dep.Partner Functions
OPSPAR3 WBS Partner Profile
OPSQ       Intervals for Cost/Revenue Types
OPSR       Maintain milestones
OPSREV  Command File Revenues (PS)
OPSS       Maintain milestones
OPST       Network Confirmation Parameters
OPSU       Maintain Activity Control Key
OPSV       Cost object ind.
OPSW      Info System Profile
OPSX      PS: Reset "Budgeted" Status
OPT1      Maintain PS Transaction Types
OPT2      Matchcode for project definition
OPT3      Matchcode for WBS elements
OPT4      Matchcode for standard network
OPT5      Customizing matchcode for network
OPT6      Maintain std. milestone group
OPT7      Project planning board profile
OPT8      Maintain hierarchy graphic / dates
OPT9      Network parameters for PS-SD
OPTA     Command file for PS -> FI area
OPTB     Command file for WBS
OPTC     Command file for network
OPTD     Command file for standard network
OPTE     Command file for PS info system
OPTF      Command file for PS graphic
OPTG     Maintain PS Info Summztn Criteria
OPTH     PS info system maintenance
OPTI      Maintain Progrss Analysis Overview
OPTJ      Maintain PS Info Unit Conversion
OPTK     Exclude Cost Elems from Avlbty Cntrl
OPTL      Maintain PS Info Comparison Criteria
OPTM     Fincl Budgeting Profile for Projects
OPTN      Maintain priorities
OPTO      Change costing variants
OPTP      Sub-network parameters
OPTQ     Maintain WBS scheduling parameters
OPTR     Strategies for settlement rules
OPTS      Maintain project version profile
OPTT     Maint. acct.assgnm.types for orders
OPTU     Maint. PS Info System field settings
OPTV     Maint. PS Info System sort criteria
OPTW    Maint.PS Info System group. criteria
OPTX     Maint. PS Info System database set.
OPTY     Capacity leveling - Time profile/PS
OPTZ     Matchcode for standard WBS elements
OPU0     Matchcode for standard project def.
OPU3     Production order control parameters
OPU4     Maintain Capacity Planning (SOP)
OPU5     Parameter long term planning PlndOrd
OPU6     Production order control parameters
OPU7     Control parameters plant maintenance
OPU8     Overview Var.: Header Line Info Sys.
OPU9     Overview Variant: Item Line
OPUA    Field selection: Network: Header
OPUB    Field selection: Network: Overview
OPUC    Field selection: Network: Details
OPUD    Field selection:Network confirmation
OPUH    Field sel: Std. project definition
OPUI     Field selection: Std. WBS elements
OPUJ     Field selection: Project definition
OPUK    Field selection: WBS element
OPUL    PS: Maintain reduction strategies
OPUM   Maintain Subprojects
OPUN    Capacity overviews
OPUO    Project schedule overviews
OPUP    Subnetwork schedule overview
OPUQ    Overall network scheduling overview
OPUR    Info System: Structure overview
OPUS    Maintain Version Number Entry Mask
OPUT    Maintain overview variants
OPUU    Maintain Network Profile
OPUV    Network Type Parameters
OPUW   Standard Network Status
OPUX    Maintain overview variants std netwk
OPUY    Overview var.:Std.ntwrk scheduling
OPUZ    Control parameters plant maintenance
OPVC    Convert Avlability Cntrl Tol.Limits
OPVP    Collective availibility checkProfile

SAP PS Reference Books:
SAP Project System, Interview Questions and Certification, Books

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