Your ad isn't showing because there's another Google ads account serving ads for the same website that has a higher Ad Rank. According to Google ads policy, only one ad with the same display URL domain can appear for any particular keyword. If two or more ads with the same domain are eligible to show, we'll display the ad with the higher Ad Rank.
What you can do
If you find that your ad often isn't showing due to this policy, you can try to improve your Ad Rank by increasing your bids and improving your Quality Score.
If you have trouble
You can always contact an Google ads representative for help.
