Watch series 2 powers itself off

Over the last week, my Apple Watch Series 2 has powered itself off in the middle of the day. I can't see a pattern in relation to the timing or any particular activity. But it is happening daily.

The only way to power it back on it to plug it into the charger.

I have tried un-pairing and then re-pairing the watch to see if this fixes the issue. That worked for a day and then the issue presented itself again.

Not sure what I should do next ... :shrug: