There are multiple reasons why a campaign may not serve:
Campaign/Ads are paused
Both the campaign and at least one ad in the campaign need to be enabled. To enable a paused campaign or ad, hover over the orange bubble at the end of the campaign or ad row and there should be a play button. Equivalently, to pause a campaign or ad, hover over the green bubble and there should be a pause button.
No funds/negative funds
For a campaign to serve, we require that you have a positive balance. To add to your balance, go to Account > Billing. The minimum amount you can add is listed from the Add Funds menu
Note: You must have at least one campaign before you can add funds.
Make sure you adjust the date range on the screen you are looking at to a time when the campaign should have been live.
Campaign targeting is too restrictive
If you are targeting specific cities, a few categories, and are restricting to only the latest OS, you are severely limiting the available inventory you can serve on. In these cases you usually will get a few impressions and not zero, but sometimes targeting and ad types (see below) will combine to make no available inventory
Not enough ad types
In general you will get impressions regardless of ad types, but sometimes when combined with the campaign targeting, you may have no inventory available. See the article on Ad Specs to get a sense of our network averages by ad type, but some categories may only serve Banner ads for example.