Have you looked at the paper yourself to see if you can figure it out? Looked at feedback from the class and apply it to your paper yourself? I am full-time faculty, but I don't waste my time debating grades with students in the situation you describe. Mostly it does not matter what I say - they respond with defensive "but...". If a student wants to discuss it with me, I have them turn in a self review/graded copy they did first and then we will schedule a meeting. If a student comes to my office hour - I give them a pencil and ask them a series of questions and have them mark it - usually they can figure it out from there. Usually after the first one or two times of doing it, they can do it on their own. The brighter/less needy ones can do it without my continued assistance and do it before they turn the paper in and get a better grade for having done so.