Not turning on when I twist my wrist

Make sure its turned on by long pressing the B button, settings, system,display, not during activity, make sure “gesture” is set to on and check the sensitivity.  Set it to high also.  If this is already done then do a hard reset by holding down the A button for about ten seconds until the watch powers off (past any notifications).  Wait a few seconds and then press the A button to turn back on. 

This was happening to me on my Venu 2 Plus.

Check your "Do not disturb" times.

I changed mine accidentally from "11pm -8pm" to "11am-8am."

It's easy to make a mistake.

This was my issue too. Accidentally did it putting on a new band. Should have turned the watch off. 

Does not work for Venue Sq 

I tried all of these, and still mine does not work. I can sway my arms around like a mad person, still nothing. This is useless, Garmin, I expected more.

Thanks to everyone here. I've been annoyed by this for aaaages. Just thought to look it up. Tbh I thought the watch was getting old and the sensor inside wasn't always working perhaps.

DND was set! I've now turned it off. Let's see if it wakes me up on the middle of the night with a bright light in my face now! Haha