Why don't notifications display in my drawer?

I'm using Oreo on an HTC U11. A few days ago, most notifications stopped appearing in the notifications drawer. I still get the notification sounds and vibrations, but nothing in the tray. It's pretty frustrating as even if I hear/feel it, I don't know which app is sending the notification! I have checked through all 'Sound & notification' settings, and individual app settings, but I can't find anything obviously untoward. I've also checked my DND settings, and it doesn't seem to be the issue. I missed a couple of orders for work, and an important message from my children's school, so would be really grateful for any assistance. Thanks in advance :)

Welcome to Android Central! Do you have a smartwatch or other similar device connected to the phone?

Originally Posted by B. Diddy

Welcome to Android Central! Do you have a smartwatch or other similar device connected to the phone?

Thank you for your welcome and reply!
No, nothing at all like that connected. It's a mystery!

Does it work after wiping the cache partition? https://support.t-mobile.com/docs/DOC-36638. The steps for the U11 Life ought to work for the U11.

Thank you so much for your knowledgeable advice - I really appreciate your help. I've now wiped the cache partition, but still have the same issue. I've just had a chain of vibrate alerts, and no idea which apps have sent them! It's making me slightly crazy tbh, I think I'm edging towards a factory reset to preserve what little sanity I have left!

Ok, try booting into Safe Mode. This will temporarily disable any 3rd party apps you installed, so if one of those apps is causing the problem, it should go away. But this also means you can't test notifications from any of those 3rd party apps -- only from preinstalled apps. HTC U11 - How do I restart my phone into Safe mode? - Support | HTC Ireland