My fēnix 5S is no longer recording my ascent or descent during activities. Can anyone advise please?

I have a fēnix® 5S. My altimeter seems to track my elevation, but when I record an activity it no longer records my elevation during the activity.

On 20th Dec it recorded my elevation during a cycle to the swimming pool. It recorded my swim, and then it stopped recording my elevation during the cycle on the way home. 

I have tried, soft reset, auto-calibrating and have updated to the latest software.

I am a regular swimmer. The only change to my activity on 20th Dec is that I started to swim a few lengths of crawl rather than only breaststroke.

Has anyone else experienced this and have you been able to resolve it?