First, you have to tell us the CAUSE of the SDCCH FAILURE RATE. Normally, you should have different indicators such as :
1. sdcch fail % due to BAD RADIO CONDITIONS
2. sdcch fail % due to SDCCH CONGESTION
3. sdcch fail % due to HARDWARE/SOFTWARE FAILURES
a.. check interferences (from GSM systems OR from external sources : military radars, CDMA, AMPS, TACS, other wireless technologies..)
b .. check ghost RACH, reduce the RXLEV_ACCESS_MIN or the RACH_FILTER for instance.
c.. check that your TRE are fine (reset the hardware)
If (2.) : check your SDCCH traffic : location updates ? SMS ? call setups ? and then increase SDCCH capacity.
If (3.) : verify your hardware / software.. change equipments, reset the BTS, etc…