I'm having the same issues with my 310xt. I get the "sync complete" message, but nothing new shows up in Garmin Connect. I had gone two weeks without syncing (though still using watch) while my computer was in the shop, so I deleted my entire history from my watch, thinking there were too many files for it to handle. But, there is not change. It still tells me that it has synced, but the activities do not show up in Garmin Connect.
On another note, I was able to transfer a training plan from the Garmin site to my watch.
Not sure why my data isn't showing up. It is making me crazy.