The search engine is part of the same system which lets you view all of your posts, via clicking on 'Profile'. For some reason, the system frequently 'times out' before yielding any results, so you either see 'Timed out' (when trying to view your own posts) or 'No relevant questions found' (when performing more general searches).
The problem used to occur about 20% of the time. Nowadays the system only seems to work about 20% of the time.
It's been reported to AB's techies dozens of times but, apart from occasional temporary improvements, it just seems to get worse.
Chris
(PS: I'll send a link to this post to the techies, so they can't say that nobody tells them about the problems!)