Maybe the updater doesn’t show the “experimental” updates. You can check this option in the Updater Applet. Right click to the taskbar icon -> configure -> Check in: Show available upgrades when backend provides them.
It seems to be fixed by unchecking “Check for updates only when system is under low load”.
This seems to be a bit of a bug, really it would be better if it aimed for a time with low load but within a limited time frame so that it would check even under high load if it had not checked for x amount of time.
Other wise you could go for weeks with a system that apparently was up to date but which had lots of security patches pending that the updater had not checked for.