Here you could check if the HO REQUEST on one side is well sent to the target MSC. If it isn’t then it proves there is an issue.
I’m sure you can diagnoze exactly when is the problem in the message flow, thanks to counter, and then deduce the exact point where the message is not flowing through.
But it’s highly dependent on the vendor implementation of counters. In alcatel, we have incoming and outgoing request, attempts and success. Really useful to see at what step the problem is.
Are you looking at the HO between just two cells, or is it the total amount of HO of one cell? I mean.. the 20% success is towards only one cell, or towards several cells?