Hello Yang,
There are 3 possibilities :
1. your TRX carrying TCH are all faulty, but not your BCCH. It’s not probable, but maybe…
2. You ANC is faulty : it doesn’t amplify the signal from 3 TRXs.
2. your BCCH TRX is connected to one sector, while the 3 other TRX are connected to another sector. That is highly probable 🙂 It might not be 1 on one sector + 3 on another sector, but rather 2+2.
Of course, this solution applies only if you have more than one sector in the BTS.