Sleep activity not being monitored on viviactive3

I am using a Vivoactive3 watch.

Sleep data suddenly appeared three days ago and showed data for two nights then stopped again. How can that happen?

I have checked that the activity tracker is ON. My heart rate is being monitored and I wear my watch through the night.

I have un-installed the android mobile app on my phone and installed it again which should have cleared any cache and ensured that I have the latest app.

Any advice please?