You can check customizing for the "Real-Time Integration" functionality via Customizing -> Financial Accounting (new) -> Financial Accounting Global Settings (new) -> Ledgers -> Real-Time Integration of Controlling with Financial Accounting.
Please make sure that the customizing is done for the "Account determination for Real-Time Integration (transaction OK17).
Please also have a look at the documentation in front of the execution button.
Plan asessment will not be passed onto NewGL because it posts on assessment account which is secondary cost element.
CO Plan data (including plan assessment) is transferred to FI using FI/CO plan-integration. For FI/CO plan-integration to work, the account used in CO must be a primary cost element. Otherwise, the system cannot map the cost element with G/L account upon creating FI data.
Two workaround are possible:
1) Use plan distribution instead of assessment
2) Manually adjust the plan data from GP12N for FAGLFLEXT