If the keywords used in the Ads does not exist in the XOVI database, the keyword will not be crawled

The ad is visible at specific times only. If for example an ad is visible each Monday to Friday between 9AM and 5PM and the crawler might crawl the keyword on a Tuesday at 10PM, the ad will not be found by the crawler, because it was not visible for it at that time

The budget for AdWords on adwords.google.com was used up, when the keyword was crawled. In this case no advertisement was visible and thus the crawler did not find the ad.