I have a bunch of rules in Outlook. Until last week they all worked fine, but now they don't get automatically applied upon incoming mail. When I manually apply rules now from Rules menu, they get executed. What might cause this?
Outlook is often flakey with rules.
Have you installed any 3rd party add-ons or applied patches to Outlook recently?
If you have the first thing I would do is uninstall anything you have added and see if the rules work again.
I have had this issue before with .pst and .ost files.
I've had to run Scanpst.exe (see here for more details on it: http://support.microsoft.com/kb/287497) in the past and it's resolved issues.
I've uninstalled patches, run the scanpst.exe tool and then re-installed patches and it's worked.
Another option would be to Export your set of rules. Delete them from your list, Restart outlook and them import them back. Here is a good article on this: