This is the marketing question equivalent to ‘what is your greatest weakness?’ The hiring panel wants a candidate that knows they have flaws but is prepared to take responsibility for their failures and shows a desire to learn from past errors.

First of all, don’t be conceited and claim you’ve never had a failed campaign. Even if this is the case, there is certainly a project that wasn’t as big a success as you had hoped. In any case, explain what you believe caused the problem and focus on how you learned from the campaign. Above all, do NOT directly blame anyone else!