Apple watch is pairing but not synching with iphone

I have a 1st gen Apple Watch and an iphone 6. (Both appear to have latest available version of OS as of writing.) They've been working great together until they stopped about a week ago. (I don't believe this coincided with an update -- although Apple sneakily pushes those when I'm not looking --, and I have not changed any settings.)

Here are the symptoms:

• The watch still successfully "pairs" with the iphone. By that I mean that watch seems to still recognize that it's connected to the iphone: if I turn off the iphone (or just bluetooth on the iphone), then the red "no phone" icon appears at the top of the watch face, and when I turn the phone back on then it goes away (or turns green if I've swiped up on the watch).
• However, no data seems to be moving between the two: activity doesn't sync, I no longer get watch notifications about new e-mail or text messages, "find my phone" doesn't work, and the time zone doesn't update on the watch when I travel.
• For what it's worth, my watch no longer seems to unlock my macbook.

How can I force my watch to sync with my phone? I don't want to lose data accumulated on the watch (e.g., activity history) since this problem. (And yes, I've tried turning it off and turning it on again... and the phone too.)

Any advice would be appreciated!

Thanks!