I know you must be irritated much by this as SDCCH Drop do affect the Call Success.
1) You must check the performance of your TREs. Most SDCCH Drop in the Network could be experienced on particulr TREs and especially when there is low Traffic i.e during the night. A work-arround is to Prioritise your TREs so that when there is Low Traffic, the TRE with the BCCH is opted for call handling.
2) If this fails, then you can delete and recreate the TREs of the cells affected