Thanks Pix for correcting me. So in our case we are having two different cases.
First case is Immediate Assignment Failure Rate which is because of BTS not receiving CHNAV message from BSC or BSC not receiving CHNAK message from BTS as I have said previously which is SDCCH assignment failure.
Second case is SDCCH drop due to T_MSRFPCI which is the case after Successful Immediate Assignment.
Thank you once again.