Sometimes publishers share a group of codes and one person puts them up on Woovit, but someone else still pulls from them to give to other people. Sorry! Woovit has no means of checking if a code is valid or not in advance.
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The first step is to let the publisher know through their contact information on the campaign page. Feel free to cc us on the request for a replacement code at hello@woovit.com

If the publisher can not provide a replacement code, let us know and we will remove the campaign from your accepted offers and restore your quota. If we see this as a pattern we will ask the publisher to refresh the codes available so other creators don't face this problem.

For some mobile platforms, codes can expire after a certain time, in one case, 30 days; this timing should have been marked on the campaign information page but the same steps can be taken above.