Show that you sought feedback from the prospect after losing, and name the specific change you made to your process as a result.
The best answers show genuine self-reflection: identifying where they lost the deal (missed buying signals, failed to engage a key stakeholder, underestimated competition), what they did to get a post-mortem from the prospect, and how they applied those lessons. Candidates who blame pricing or politics without self-reflection are concerning.
Reveals self-awareness and coachability. Everyone loses deals. The question is whether they learn from it. Ask: "What do you do differently now because of that loss?"