That’s been messed up for quite awhile now. A couple years back the forum had a meltdown and was down for awhile. Before that you’d get email notifications within a few minutes of a reply. Right afterward the notifications would be delayed by days, literally. Nowadays I get the emails anywhere from 1 to 6 hours late. Try turning notifications off and on again.
There is a sub-forum for forum issues. You can post there about not getting notifications.