This seems to be happening to a LOT of FTM users lately, myself included. A similar discussion can be found at http://boards.ancestry.com/topics.software.famtreemaker/9503...
For most of the people reporting the same problem, sync failures started occurring on or after 22 Feb 2013, as was the case for me.
It doesn't appear to be a problem with the FTM software, rather an issue on the ancestry.com side. It's very frustrating. I make nearly all of my tree updates either through the ancestry.com website, or through the iPad app, synching with Family Tree Maker primarily as a form of backup (and to make use of report/publish features of FTM).
We're asking ancestry.com support for some kind of acknowledgment of the problem, that they are aware of it, and are working on a resolution to the problem. I've been checking back daily and attempting the TreeSync, just to see if they've magically fixed the problem. No joy.