iPhone not ringing when receiving texts, even with notifications on full volume

I had the same issue when I first bought my Apple Watch. I finally figured out that if I turn OFF the Bluetooth setting, then both devices ring. I only turn on Bluetooth when I need it for something specific and all iOS updates turn it back on so you’ll need to remember to switch it off again

Had same problem, if you can open the text that did not ring go to the top where the number is located touch that number another window will open there should be a line that says hide Alerts make sure that is not activated.

If your iPhone is locked, you will receive notifications on your Apple Watch, and if your iPhone is unlocked, you will receive notifications on your iPhone. You will not receive them both places.

You can disable Wrist Detection, then all notifications will be delivered to both devices. But this will exclude various features, so I cannot recommend it.

From Apple Support:

If Wrist Detection is off, you won't get Stand notifications, and your Apple Watch can't track your Stand progress. Background heart rate readings (like resting and walking rates) won't be taken if Wrist Detection is off. 

You can find more information about this here:

Notifications on your Apple Watch - Apple Support