Describe a time a campaign was underperforming. How did you diagnose and fix it?

Show a diagnostic order rather than a list of tactics — that is what separates someone who has run accounts from someone who has read about them.

Check in this sequence:

• Is the tracking correct? Before concluding a campaign has failed, confirm conversions are actually being recorded. A broken tag makes a profitable campaign look dead, and this is the most common cause of a sudden collapse.
• What changed, and when? The change history shows edits; a sharp break points to a change, a gradual decline points to competition or seasonality.
• Where in the funnel is the loss? Low impressions is a budget, bid, or targeting problem. Good impressions with low CTR is an ad copy or relevance problem. Good clicks with no conversions is a landing page, offer, or audience-quality problem. This one distinction directs everything that follows.
• Check the search terms report. Irrelevant queries draining budget is extremely common with broad match.

Then fix, and change one significant thing at a time so you can attribute the result.