First, if it's a craigslist ad missing, please see this answer.
Otherwise, the most likely explanation is that you've set an option such as maybe max mileage or body style, which is filtering out the result you're expecting to see.
It's also possible that the car is listed on one of our comparison sites. We pull in results from as many sites as possible to show right on our results page. There are still a few sites we can't (yet) pull from though, including Autotrader and craigslist. We'll still help you with those though, by providing comparison links, allowing you to see results matching your search on those sites with just one additional click. We have no way of knowing beforehand whether those sites will have matching results for your search though, so you will have to open the links we provide to check.
If you still can't find the ad, please feel free to get a hold of us, and let us know which ad you expect to see but don't. We'll look into it ASAP. Please include the URL (link) to your AutoTempest results page, so we can see exactly what you're seeing.
Finally, please note that neither AutoTempest nor our partner SearchTempest is affiliated with or endorsed by craigslist.