A few days ago my Edge 510 quite syncing with the Connect app on my android phone. The phone would connect, and the 510 would show uploading ride and upload complete, but the ride would never show up. If I watched in the app I would see a "sync failure". The ride would upload fine in the desktop app but would show 4 errors in the sync which were 4 successive rides from over 3 years ago.
Garmin Express walked me through how to remove those files from the computer, but it does not tell you how to remove them from the 510. This isn't hard, you just have to view the 510 in your Windows file manager, navigate to the Garmin\Activities directory, and look for the files which match the date of the erroneous files, Delete them and make sure you also delete the same files which will be in "C:\ProgramData\Garmin\CoreService\Devices\<device 10 digit ID#>\Sync\FIT_TYPE_4\YYYY-MM-DD-HH-MM-SS.fit