It shouldn't have anything to do with 2012, they would sync into their own databases (2012 would update from Ancestry to its copy of the database, and 2014 would update from Ancestry to its copy; changes in 2012 would go up to Ancestry, and 2014 would download those changes, just as if you had made them ON Ancestry). The reason for this is, 2014 had to convert the old database; they aren't sharing one (which could have caused problems).
My 2012 worked fine too. The 2014 seems to have some sort of bug ("undocumented feature," heh) where something is simply conflicting. I would have hoped to have seen an admin response to this issue by now, and I'm quite disappointed that I haven't heard of any plans to address it! Many bugs aren't as large an issue; being completely unable to sync ANY changes you make in FTM makes the software nearly useless as anything other than a viewer and report generator. That defeats 60% of its design purpose!