A new, approved campaign should start receiving impressions within about 5 minutes. If it hasn't started by then, this is a real setting issue, not normal ramp-up.
Start here: run Campaign Health Check on the campaign. It checks everything below in one pass and usually names the blocking issue immediately — which saves you changing three things at once and not knowing which one worked.
If you'd rather check by hand, in order:
1. Bid too low — the most common cause. If your bid isn't competitive you simply don't win auctions. Watch for the zero-bid alert and bid hint on the campaign, which tell you when your bid is below what the traffic is clearing at. See How do I fix a "Low Bid" indicator?
2. Balance is $0 — a correctly bid campaign won't deliver on an exhausted balance. See Balance alerts and running out of funds.
3. Insertion Order expired — an expired IO stops delivery silently, while the campaign still reads as Active. Check the IO dates.
4. Creative not yet approved — creatives are reviewed within 2-15 minutes; check status before suspecting the campaign.
5. Targeting too narrow — very restrictive geo, device or source targeting can cut available inventory to near zero. See What targeting options are available?
6. Campaign on hold or budget exhausted — check it isn't paused and that the daily/total budget hasn't been reached.
Still nothing after 5-10 minutes? Contact support with your campaign ID and what Health Check reported.
You don't have to check manually: if a live campaign gets zero impressions for 24 hours, we alert you in the notification bell and by email. See Notifications and email settings.