It sounds like you're Messages is behaving as if it is on Do Not Disturb for one contact when it shouldn't be. Not being able to get timely alerts from your friends and family is not a way to get the most from your iPhone. But rest assured, I'll do all I can to help. Just to be sure, we cover all potential aspects of this, have you tried to restart your device since this issue began? Also, what happens if you force close the Messages app and re-open?
Check out these links for these steps and let us know if it helps.
Restart your iPhone, iPad, or iPod touch
How to force an app to close on your iPhone, iPad, or iPod touch
Restart your iPhone 8 or earlier, iPad, or iPod touch
- Press and hold the Top (or Side) button until the slider appears.
- Drag the slider to turn your device completely off.*
- After the device turns off, press and hold the Top (or Side) button again until you see the Apple logo.
*With iOS 11 or later, you can turn off your device by going to Settings > General> Shut Down.
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