yes, exactly, check the message flows, for outgoing HO (for the serving BSC) and incoming HO (for the target BSC).
You can find the message associated to which timer.
In the same way, you can associate QoS Counters to the message flow. Check where is the discrepancy in the QoS counters, and you’ll see exactly which message is missing.
That will lead you to the equipment which is not performing its job correcly (and which wasn’t configured properly 🙂 )