HI
Have you SDCCH congestion? or SDCCH assign fail BSS? or TCH assign fail BSS?
If cssr is low due to SDCCH congestion and your sdcch dimensionning is good then you may have A interface congestion if you do not have HW/SW problem at BSC or TC level. Check if you enough CIC on Ater link or blocked CIC or N7 instability.
Good Luck!!!