I have been using Garmin Express fairly seamlessly for months with my 910xt. After adding a FR220 to our household, Garmin Express tried to upload my 910's data to the 220's account. I got this straightened out, deleted the 910's activities from the 220's connect and tried again. Express said the 910 was syncing, yet when I viewed on the connect account, no new activities show up on either connect account. Very frustrated here, and any help would be greatly appreciated. Running windows 7, 910 device software is up to date, Garmin Express is also up to date.
Try resending all of the activities on the watch. This will cause the watch to transfer all of the activities even if they are marked as being already sent on the watch. To resend the data from your watch:
I'm betting that you got yourself into a situation where your 910 is associated with a different account. In Garmin Express, select the 910XT from the dropdown on the left, then click the Device link on the right. Scroll down to the Garmin Connect Account option and click the Change Account button. Re-enter the account credentials for the account you want to upload your activities to. Once you've done this, create a short test activity, and try to upload it. If that works, do the Transfer All as suggested above to send your recent activities to your account again.
I am having the exact same issue with a 910 xt. I have tried all of the tips on this post and still no luck. I have an Edge 500 and Vivofit and they sync with Garmin Express, no problem.