Pictures Folder Sync never ends

I have ATIH on two PCs which are on my home network which also has a NAS drive connected. Both PCs are running non-stop backup.

I also have 3 sync jobs set up (Docs, Music, Pictures) which synchronise folders on both PCs and the NAS Drive.

Over the past three days the Pictures sync on one of the PCs has been running constantly. Each time it completes it restarts and then runs for several hours before completing and then restarting. This prevents the other PC from synching as it is waiting for the first PC to complete. I have tried having only the first PC powered up but the problem still remains.

Any ideas what could be causing this, it seems like some kind of loop?

Thanks

John