my garmin edge 520 has suddenly stopped recording rides

Just in the last few days it appears that my Garmin Edge 520 has stopped recording the data from rides.

Everything looks fine when I start, proceed, stop and save the rides as always. However, I've just synced with Garmin Express and certain days have just not appeared when I go to Garmin Connect.

I've just been onto the activities folder on the Garmin connection on my mac and all rides are there. Rides that normally appear on Garmin Express are usually between 98kb and 200kb. However, the days that don't appear on Garmin Express are 2kb.

Can someone please explain what the issue may be and how I can rectify it please?

Peter.