This is a very old question and possibly no longer relevant, but since I noticed the same phenomenon on one of my forums, I have thought about this and concluded that the thing here is that in this specific case of a “slow forum” digests become more or less indistinguishable from notification emails and that’s why you might complain about people not getting notified about posts.
So the way to solve this is to make sure people understand that they need to watch relevant categories (or just make them watch by default). Digests are digests and doing their job as digests just fine.