I contacted Garmin Product Support but they were of little help.
I ended up searching the forums and found a solution (of sorts). I removed my Descent MK1 from Garmin Connect Mobile and added it again. Syncing now works. Unfortunately, I lost all data from when syncing stopped working, which was a whole day's worth of steps, etc. and any of the detail (heart rate, etc.) for the current day.
I have the same issue...when I finish an activity, although I sync it and Connect Mobile sends the "You can see your activity" message, I cannot see it. Today I can see sleep time only until 2 AM although I woke up at 6. Tried to sync at home through wifi and nothing...It seems like there is an issue here.