Cause : always a hardware problem in the BTS
If path balance positive (above 10dB), then it is the downlink path which is faulty (the Tx path)
If negative, then it is the Rx path.
But of course, that depends on which formula you are using, it is not standard.
Anyway, try changing your combiners, your low noise amplifiers, your TRX modules and check your antenna – BTS feeders / connectors.