In my case, manual update checks work fine (v5 free), both for engine/defs and for program, but there's been no auto updates at all since I upgraded from 4.8. Can't find any error messages that would give me hints, looked in system Events as someone had suggested but nothing there jumps out at me.
I've switched back and forth between "always connected" and "dialup" settings, also have tried both no proxy and auto-detect with no success. Sounds like, from other postings, trying a Repair is the next step.
(Edit, shortly later) Bingo! Repair did the trick ... when I opted to look at the resulting setup log, it appeared that nothing had been added or changed, but no sooner had I finished than I got that good ole pop-up notification.
