There’s an occasional bug that stops the history transferring to PC; goes back quite a while (was an issue for me last summer anyway). It seems to only happen when I do a lot of rides without connecting to a PC.
If you connect to PC then go into the folder structure and delete the oldest folder in the “Laps” folder (I think this is in the “history” section, but I don’t have my Garmin in front of me) then the history and data for the subsequent rides will transfer. You loose the datapoints for the one ride that was stored in “laps”, but it holds total distance/ time/ kCal data for that somehow (that is using Training Center – haven’t used Connect).
For avoiding the bug, resetting the GPS before switching off the unit or connecting it to a PC seems to stop it happening if it’s the same bug I was getting (reset by holding lap for 3 seconds, not a hard reset!).
EDIT: Backing everyting up onto your hard drive before trying this isn’t a bad idea!