Ever since I bought my Forerunner 310xt a couple of months ago, I have had intermittent troubles syncing it through Garmin Express. I tried everything I could think of and every solution I could find on line (except for the master reset, which was completely impractical after the first couple of days). First the piece of crap was not recognizing my ant stick, then in could not upload the data to Garmin Connect, then it said it was synced but my data wasn't showing. Until last night, my last successful sync was on April 22. After countless hours of trials and research, I uninstalled Garmin Express and installed the old Ant+ Agent. At least I could see it was trying, instead of just sitting there silently mocking me, and I could see it got stuck on a specific activity midway (or perhaps the connection timed out). I then deleted all "old activities" (older than a month and already synced) and yes! it worked. Now I can delete all the activities since I actually have them in Garmin Connect and take it from there. Mind you, I don't know which of these empirical steps actually solved the problem and I don't know if they will work for you, but at least they didn't involve a master reset.
Personally, I am done with Garmin Express until the syncing issues are solved.
To the Garmin Express Customer "Care" - a master reset is not, I repeat, IS NOT an option when you have several activities, or even one good activity, already on the watch. Just as a reminder, the reason I bought my Forerunner was to keep track of my progress, not to tell me the time of day. The missing sync button is a major irritant since Garmin Express does not, I repeat, DOES NOT automatically sync when it detects my device. And it seems to be crashing my device every time it actually connected.