Why do I never have the perfect comeback when I need it?

This morning I reluctantly drug myself to Weight Watchers. (Yes, I am still going every week, although the last few weeks have not been too good.) Anyway, as I said, I drug myself there to weigh in today and I got there as they were putting things away so they could close up and go home. They were still open however, so it's not like I just slipped in as they were closing the door. I went to the pre-paid box to find my card and the lady at the counter said "we have the cards over here". So I walked to the counter and she asked my name and when I told her she said "that's the box MaryJo has." Well MaryJo just sat there and continued filing cards into her box, so I repeated my name for her and when she found it she said "do you want it delivered air mail or do I have to walk it to you?" Huh? So then she tossed it to me across the counter, kind of like she was all put out about it. So I stand there for a minute and finally say, "well, you still have to get up to weigh me." Duh!!! So she gets kind of huffy and gets up and walks to the scale. She weighs me and says that I was down a little bit. Then she adds, all snotty like, "try to keep it in that direction." And wouldn't you know it, I couldn't think of anything to say back. Oh, I can think of a dozen things now, but at that moment, my mind was blank. Why does that always happen?