Between 65% and 85% there is not much change : the cell is just not working. It wasn’t working, and it is still not working.
To fix this problem, you can already assume that the problem lies in the Hardware.
Try allocating the SDCCH TS in one TRX only. TRX1, then try in TRX2, then in TRX3, and so on. In each case, perform a call test and see if the problem is better (or just check SDCCH Assign Fail BSS %)
Try also deleting the cell and recreate it from scratch… also try resetting the TCU/TSU.