ok, you need to investigate further.
1. drive test : are there HO COMMAND sent to the MS when going from BSC1 to BSC2 ?
2. QoS counters or Abis+A interface traces (on both BSC’s) : can you plot the inter-BSC message flow and point out which message is “failing” ?
For instance : does the BSC1 send a HO REQUIRED to the MSC ? Does the MSC sends a HO REQUEST to BSC 2 ?
If you find which message i missing, then you know which equipment is not doing its job properly. Then you know where to check the configuration 🙂
interBSC message flow (p.37) :