Are you running the corresponding beta on your watch?
FWIW, iOS11/Watch OS 4 is notorious for its connection issues - leading to battery drain on both devices and making it hard to upgrade to the latest watch betas.
i'm having the same issue and assumed it was simply becasue I'm on iOs11 on the phone but watch is still on current OS. I'm nervous about updating the watch to beta at this point.
Are you running the corresponding beta on your watch?
FWIW, iOS11/Watch OS 4 is notorious for its connection issues - leading to battery drain on both devices and making it hard to upgrade to the latest watch betas.