Our team manually reviews new creatives for approval during normal business hours. If your submission is rejected, we'll send you an email letting you know why. You'll also see the same explanation in the status column of your dashboard:

To find out why a creative was rejected, click the speech bubble icon next to the "Rejected" status on the Creatives tab.
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If you've resolved the issue, you can resubmit the item for approval by clicking "Resubmit for Approval" and selecting the edit you made from the dropdown menu that appears.