Heart Rate 'graph unavailable' for outdoor walk

Hi,

For the activity type 'outdoor walk' the heart rate graph is always unavailable afterwards. For other activities such as rowing there is no problem.

This has been going on for weeks, one day I row, the next I walk. None if the walks get recorded, all the rowing does. The watch fits tight enough.

Apple watch 4, iPhone XS max. latest updates, restarted etc.