Ads stop showing after a day or so

I set up my advertisements and get them to display correctly. However, after a day or so, all of the ad simply stopping appearing. The only way to fix this is to clear the caches, which fixes the issue temporarily. I tried following the debug instructions by visiting the serve.php path for one of the broken ads and found this error:

document.write('\x3c!-- No active ads were found in 0 --\x3e');

Any ideas about what could be causing this?