Are you sure there is no interference nor coverage issues? What about congestion in the target cell?
What about the ho success rate in the other direction?
Were you able to detect which adjacency was degraded exactly ?
Perhaps all other neighbors are fine, just one of them is degraded. There is perhaps 0% success rate towards one main neighbor (0 success / 1500 requests) and 100% towards other neighbors (500/500)
If all this has been done, then you can assume there is a soft/hardware problem 🙂 good luck…