PitotTube,
Not ofthen that I agree with WWW, but this is a rare exception. As redsnail says, use that failure as a learning point, something that you can build on to improve where you went wrong.
Finding a cause for the failure will just not have the desired effect, and will not generate that (probably) small improvement that the examiner is looking for.
Be positive, ask questions, and put it down as part of the learning process (and life for that matter).
Keep enjoying it too.
S