iWatch 6 often won't wake up.

I have an iWatch 6 with software 7.6.2. ,(the latest update I think). Quite often, my watch will not awaken even if I raise my wrist or rotate the crown button . I usually get round this by taking the watch off my wrist,giving it a shake + reenter passcode. Not very convenient just to look at the time is an understatement.