I have the exact same issue, apparently already for months, since day 1 I purchased the apple watch series 2.
I was so stupid to believe my watch worked perfectly, just because he mirrored my iPhone notifications.
Now I wanted to use an 3d party app (FunGolfGPS) which does not connect to my iPhone. Googling for 2 straight days for a solution now and getting sicker of this bug as we speak. I tried everything as mentioned above, on other forums.
Result is that now I don't even have my contacts synced anymore and in no way they want to resync.
It is such a HUGE shame that a company like Apple isn't able to resolve this issue.
I can try to contact support, but based on other forum posts that won't help.
I can try to go to an Apple Genius Bar - there is one in 150 km from where I live - but who has the time for that ?!
So the last option will be using my 600$ watch as an overpriced time-showing device with some notifications I can see on my iphone screen as well. Thank you Apple