Using the Troubleshooter
Click the "Troubleshoot Campaign" link at the top-right of a campaign's Campaign Settings, Creatives, or Keywords/Targets tab to see a checklist of potential issues. Anything blocking the campaign from getting traffic will have a red X in the "Status" column and more details about the issue in the "Issue" column.
NOTE: If your account balance is almost depleted, your campaigns may not have enough funds to run. Similarly, if your campaign has almost reached its total budget, it may stop running. The Traffic Troubleshooter may not report these as issues preventing your campaign from receiving traffic. Adding funds to your account and/or raising or removing your total budget will resolve these issues.
Finding Unidentified Issues
When the Campaign Troubleshooter doesn't report any issues, your campaign is live and eligible for impressions. If a live campaign isn't getting traffic, it could mean it isn't optimized, hasn't had enough time to get impressions, or you may be looking at a date range without volume.
Learn more about why live campaigns don't get traffic, and check out these best practices for increasing volume.