Dear New
i hope you are new, for Low TBF establishment success and at the same you are suffering from TBF drops it indicates either the radio condition are too bad ==> check the CS performance of the same cell if it’s also have the same problem SDCCH Assignment failure are high or you have high tch drop then you have to check the frequencies, if not and you have a dedicated TRX for PS then you may have faulty TRX which is the second reason.
also you may have wrong parameters setting let me check this issue, but to check you have tell me which vendor you are using in your network