check your antenna system, there is one of the 2 feeders for that sector which is not transmitting signal to your BTS properly.
So check, frombottom to top:
0/ vswr on each cable
1/ bts internal cabling tx and rx
2/ jumpers from bts
4/ couplers / diplexers/triplexers
6/ jumpers to antenna
the idea is to find an inversion somewhere.
you can also check sdcch/tch allocation success rate for each trx, see if some trx are poor. In such case, check other sectors too. if other TRX are poor too, then you can assume there is a cross jumpers issue.