This is because you've reached the trackpoint limit (13,000) on your device, so the map data is being overwritten, leaving only summary data.
Okay, that explains why I can't upload from my Forerunner, but not why I can't upload a file that I've exported from Training Center - or at least why some upload and some don't.
Your activities will be migrated even after uploads are disabled. Can you post a link to the MB activities that are problematic?
Windows or Mac? At what point in the upload process are you getting stopped? Any error messages? What version of Garmin Communicator are you using?
I'd really rather not go that route because there are now several workouts that I have not uploaded yet and getting rid of the history will delete them out right.