I've been getting them in bunches every so often instead of instantly like I used to. I was going to look into my email settings but I may not be my settings after all if others are having the same problem. I don't check my post count so I have no idea if it is frozen.
They're not real time, there's considerable delay. Same thing started happening on another forum that runs the same base as you, not sure what drove it initially but after an upgrade there they've not been the same. Notifications are days after the fact.
I noticed a group of notifications showed up the other day and were quite old. It could have been a lack of being observant on my end though. I in fact responded to your PM yesterday (am), see if it shows up.