Recently I had this issue and discovered that my Notifications Messages was turned off. So some how either I, or some app had turned it off for me and I was not getting notified of messages being sent to me.

Also you can click on the word Messages and make sure Show Silently is not turned on as well.

Hopefully this will solve your issue as it did mine.

All the best from
James M Sandbrook.
23rd of October, 2021.