There might be several reasons for that.

For instance, your advertisement might be rejected or not yet activated (it takes up to 4 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 a daily budget of 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. You'll probably see the reason yourself, or you'll find an event that clarifies things. If you fail, please, contact us - we'll gladly help you.
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