Hi Upendar,
Please check the followings:
OKB9 assignment:
If it is done by profit center and your entry exists in OKB9, then check profit center in material master data at Plant Level 2.
If it is done by valuation area and your entry exists in OKB9, then check Plant in delivery.
If it is done by business area then check BA in your delivery.
You can also try momentarily populating cost center as default in cost element to know if cost center assignment is working,
Best Regards!