Multiple reasons:
1. Your daily budget has been spent
2. Your bid is too low for the ad to show on first page
3. Your credit card has been declined
4. The ad was disapproved
5. You are not in the location you are targeting
6. Ads are set to deliver during certain hours
7. You keep searching for your own ads
If you repeatedly search for your own ad using Google Search but never click it, you may stop seeing it. That’s because Google’s system detects the IP address of your computer, and stops showing you ads that it thinks you aren’t interested in. Remember to use the Ad Preview and Diagnosis tool to view your ad as it appears in Google search results, instead of performing a regular search. It prevents you from racking up impressions while you try to locate your ad.