Google are now incorporating your personal search and click history into the display of adverts. You are likely to see this manifest itself as your ad disappearing when you carry out your searches. If you do the search a lot and never click on your own advert (sensible behaviour!) they will stop showing you the advert, figuring that it's obviously not relevant to you.
I just got off the phone with Google AdWords support after seeing this happen on one of our client accounts.
We are keeping a close eye on this account because we have just taken over the management of it and have made some changes to the account including adding quite a few negative keywords (since they do a fair bit of broad matching). They have consistently been appearing very high up for their highest-traffic search (very often in the highlighted box at the top of the search results). Today, despite their CTR having improved on the historic levels because of our negative keywords and an improved advert, they were nowhere to be seen.... Most of the time. Every so often, they would appear - exactly the kind of behaviour you would expect if their budget were too low compared to their CPC (but that isn't the problem - the budget is set at more than double what they have spent on recent days).
I was a bit stumped (especially because the preview tool showed them appearing).
When I spoke to the account manager at AdWords, she told us that there has been a change to the algorithm to take into account your history with respect to the particular ad. Apparently this change has only come in the last few days - and I haven't seen any mention of it (have I missed something?). I can't see it on the official blog.
The official advice is to trust your stats and reports and to use the preview tool as that is what everyone else sees. I'm worried though, that if they don't publicise this a little bit, advertisers are going to want to increase their bids figuring that they aren't paying enough and their keyword must have become more competitive (this is what our client wanted to do). I think Google should let people know about this kind of change so that advertisers aren't shocked when they see it happen.
The logical extension of this behaviour, by the way, is to stop showing you any ads if your history shows you don't click on them - you're obviously not interested! I can't imagine they're going to go there though...