Time Machine verification

My Time Machine backups seem to be working correctly, in that it runs backups and I can enter time machine to see files in the backup. However, it regularly stops during verification and always at 74%. I have left it verifying for upto a week, but once it is stuck it never moves on until I skip verification and then it will work okay for a few times.

Is there a way of finding out what is happening and if there is an issue with the backups? I really don't want to delete the backup files and start again if I don't have to as I have backups going back a couple of years.

I am using a 2011 iMac, with High Sierra and a gigabite ethernet network connected via an Airport Extreme to the Airport Time Capsule.

Thanks