I have had several conversations with ancestry.com tech support on this, and they say there is no way to "force" a detailed comparison that would find all items not in sync and allow a "proper" sync. If you only have a small number of media items you might be able to find the missing ones manually, and manually delete from FTM, re-sync, replace in FTM and then re-sync again to get them back. Their main advice is to break sync, and re-upload and link a new paired tree from FTM to ancestry.com. I had too many, and just gave up and did a fresh upload and link. Now I have 2,000 hints on ancestry that I had previously examined and either accepted or ignored, that I have to do all over again, and many invitations to re-issue.