does this happen even after you've read all outstanding PMs?
I found in the past, for some reason, if I was receiving a large number of PMs in a row and I hadn't gotten round to reading them I wouldn't receive other notifications until I'd read them all and then it would go back to normal.
Also you might want to try logging out of ALL places you use the forum (using mobile access one always forgets to close the browser properly!) and then log in again.
Just a couple of thoughts