Interesting. I never get this - perhaps a delay of a few hours, but never days.

The only time this would change for me is when I'm using a moderated group and
the moderator is unable to get to the messages for a couple of days.

Next time this happens to you, it'd be worth sending a "did you get the delay
too?" to the group involved. If they didn't, then there may be some problem
with your e-mail chain of delivery.