Diff backups failing for months, no solution

I've had a support case open for several months regarding the same issue. Our primary file server will not run a diff backup. I can always run a full backup however the diff backups fail. Thinking of switching products, however we haven't had this product for even a year. Tried repairing the backup software, deleted and recreated the scheduled job etc.. nothing seems to resolve the issue. Using the latest version of the software. The vss writers all show as being stable and having No error. The diff backup will run and shows as accessing the file share in Process Monitor and then it just stops without any errors. The job hangs at 15%, I have to then stop and start the services on the client machine. Has anyone else experienced anything like this or have any ideas?