the first step is to check the QOS stats:
1/ are there HO REQ and HO SUCC between those 2 cells, over the last days ? Check hourly. Check both ways.
YES -> go to 2/
NO -> go to 3/
2/ Is the target cell congested ?
YES -> in such case, the incoming HO are rejected. Tune the target cell in order to reserve one or two timeslots for incoming HO.
NO -> got to 4/
3/ Are both cells in different BSC, or different LAC ?
YES -> Check the core network configuration
NO -> delete & recreate the relation
4/ You need to perform Ater traces and A traces in order to understand at which stage the HO doesn’t work.