I have the same issue and spent over 90 mins with Apple support yesterday (I spoke with a supervisor).
First, I will detail my issue so that everyone that is suffering can add to this serious problem:
- When an iMessage is received it should light up the lock screen and appear as long as the "show on lock screen" is selected. Additionally the phone may vibrate if that is set in your notification settings. After upgrading to iOS 4, this is not occurring. When you wake up the screen the notification is on the lock screen so the iMessage has arrived, the notification just isn't working.
- If you have a connected device - in my case a Garmin watch, the iMessage notification is pushed to the watch so the push notification is working, it is just not working on the phone. I tested with bluetooth off which disconnects the phone and the same issue occurred so to be clear, it has nothing to do with the watch.
- I switched off all iCloud connected devices and computers which made no difference
- I do not have iMessage in the cloud switched on
- I have no issue with SMS messages which are pushed to me as they should be
Next...my conversation with Apple support and how you can possibly - and temporarily - fix the issue
- Frighteningly, they are unaware of any issue so please please call them and file bug reports if you can because the sooner they recognize it, the sooner it'll be fixed! I had two Apple reps get confused around the preview settings on iPhone X. The setting that causes the phone to light up when an iMessage is received is the "show on lock screen". The show preview option just determines whether you see the first few sentences without having your face "unlock". Switching between these does not affect whether an alert is pushed to you or not, just the behavior once that it is (pushed)
- That said, switching between preview modes seems to cause the iMessages to intermittently work correctly. BUT, it doesn't work more often than it does so it is not a solution
- I switched the phone off and back on again: no change
- Last I logged out of my iCloud account and performed a hard boot. This solved the issue for the rest of the afternoon but it returned in the morning.
Next steps for me:
- I am going to perform a hard boot again (details below) and I will switch off do not disturb overnight to see if that solves it so that it doesn't return
To perform a hard boot:
- iPhone X: Hit the up volume button, then down volume button and then hold down the power button until the whole phone turns off and reboots
- Non-iPhone X: Press the home and power buttons simultaneously until the phone turns off on its own and reboots