There might be several reasons for that.

For instance, your advertisement might be banned or not yet activated (it takes up to 8 hours to verify and approve the ad, although usually ads are moderated rather quickly).

Another possible reason might be that your daily budget is too low. Campaigns with daily budget less than 0.00000100 won't spend.

Yet another possible reason might be that your campaign doesn't contain enough traffic sources. Also it could be a bug.

First of all, please, check your campaign page. Probably you'll see the reason yourself or you'll just find an event that makes things clearer. If you don't succeed, please, contact us - we'll gladly help you.
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