I've got the same problem, my Edge 500 stopped recording calories a few weeks ago (on August 28th) and that reading is nothing like previous figures for similar rides; it shows 1102 calories burned for a 14 mile ride!! The same distance a week earlier was 440 calories. I've seen a few threads on here relating to similar issues but no solution. Anyone from the Garmin team monitoring this thread? Any suggestions? As far as I can tell, a re set (full or partial) doesn't solve the problem.
Deleting my old activities worked for me. If found that solution onthis thread .
My Garmin Edge 500 wasn't anywhere near capacity when I did this but that was all I needed to do to get the calorie count working again. I think I adjusted my own profile on it, my weight and so on, but I don't think that was part of it.
Check you User Settings. They can be found in Settings: User Profile: Activity Class. Mine keeps resetting back to an Activity Class of 10 after syncing. I am not a 10 but when it does this I don't get a calorie count. Just change it to another number. I don't think it matters to make it start working but you should change it to an Activity Class that actually matches your riding. I happened today and I didn't get data for a 55 mile bike ride. This only started happening to me when Garmin introduced the "modern" version of the website. Guess it is Garmin's way of telling me to upgrade to something newer than my Egde 500.