I am having the same problem with a brand new 910XT and the most recent software. I guess it is time to return this watch before it is too late. My Forerunner 10 was far better.
As moderator mentioned, you may find more information in the Garmin Express forum. However I also have the 910 XT and was having the same problems you mentioned. Here is how I ended up fixing the problem – while my Garmin Express was up-to-date with the latest release, I ended up completely removing that program from my computer doing a uninstall (windows 8.1). This was necessary also for me because my computer stopped recognizing the ant stick. I then downloaded a fresh program file of the Garmin Express and installed that. Now my computer recognized the ant stick. I added my 910 XT and bingo. Now my workouts load to Express, and then to Connect. One thing I also did in trying to troubleshoot this I ended up deleting the history of workouts from my watch figuring that perhaps I had a corrupt workout on the watch. I am not sure this was necessary. And note I had these older workouts already in Garmin Connect before having troubles with uploading recent workouts. My troubles seem to have begun about March 20th. Hope this helps. Besides this software issue I've had great luck with the watch itself.
