I am not an expert in BTS and channel concepts. But i could suggest simple steps to troubleshoot. As u mention that the problem is with a single BTS, the following things can be performed.
1. Re-integrate the cell to the BSC.
2. Verify the frequency plan.
3. Mainly check MAIO,HSN and ARFCN.
4. It could be even due to faulty BTS equipments.
5. Finally verify the Abis path.