I'm having this issue too - STILL (iPhone X, iOS 11, up to date.) It's affecting text messages and phone (my phone doesn't ring, but I do usually get a voicemail notification - not much help...) It's about 90% of the time; with text messages I don't get a banner, a sound, or anything, just a badge.
My notification settings are all as they should be, including checking the sounds; DND is turned off, mute switch also off. I tried turning off bluetooth - magically the text notification worked again. After turning bluetooth back on, no text sound.
Fixing it by turning off bluetooth is a huge problem. I use my bluetooth for my watch, my echo, my headphones. Has anyone heard anything about a fix from apple? It's really frustrating.
What's even more annoying is that my phone just doesn't ring. Ever. There I am waiting for important calls and.... I get a voicemail notification. Brilliant....
edited: I also read this at appletoolbox, which I guess solves the question about why bluetooth is the fix. But it's annoying - my watch is usually on silent but with haptics, and I think it usually does buzz for messages (although it's hard to say for sure as I just can't remember)
"If your iPhone pairs to an Apple Watch, by design your paired iPhone does not chime when a new text arrives. Instead, Apple routes your alerts to your Apple Watch, especially when the iPhone is locked.
As of now, there is no option to play a sound alert on BOTH your Apple Watch and your iPhone at the same time. It’s one or the other, based on the locked status of your phone. The rule of thumb is if your phone is unlocked, notifications go to the iPhone. If your phone is locked, notifications go to your watch."