check your qos statistics, you shuld see when is the failure occuring : during the ho preparation or during the HO execution.
then, as optimizer said, check towards which target cell the failures are occurring.
look on a map where is the locations where subscribers are likely to make a HO.
perform a drive test there.
if all radio conditions are good, but HO just don’t happen, then the problem might lie in your equipment configuration database. Try erasing the neighbor relationship and re-create it.
delete the cells and re-create them
then you can assume it is a hardware/software problem in one of the boards. this is an advanced troubleshooting, so good luck if you reach that stage 🙂