Hi Sankar,
How are you updating the BP ?
If its GUI, then this thread might help! :
BADI to check and update field in transaction BP through WEB UI
Please try the code given at the end of the thread.
While if its through Web UI, I think by changing the current entity at the start of EH_ONSAVE in BP_HEAD Overview Page will solve your problem.
Regards,
Devashish