Intermittent notifications

Why is it that the notifications being the little red number that pops up when there is a reply, only works for certain threads and not all the time.

There have been replies to my threads and it didn't say anything and later someone will ask if I saw the reply and I didn't. It works on some replies but not all, even within the same thread.

It didn't notify me I had feedback.

When anyone replies to the coffee thread, it pops up as a notification like maybe once every 30 pages or so.

Is it something I'm doing?