You must investigate further by looking at all the messages that should be sent and received in case of inter MSC HO.
Find out which message is not received, by which equipment, and you’ll see where is the misconf.
Here are the interfaces you should monitor :
Air interface (with drive test)
A interface (with protocol anlyser or with BSS or NSS stats)
MSC-MSC interface (with prot analyser or with NSS stats)