It is the NR policy not to tell the people testing why they had failed. If you are telling these students this, you could get yourself and the program in huge trouble.
Reasoning why, in my opinion why you are not told, is to make you THINK. "Damn, what did I do wrong? What did I miss?" You don't know, so now what? You go back and STUDY EVERYTHING AND GO OVER IT ALL! If you forgot to check a lead after a rhythm change, why should you be told this? You should know better! If you aren't told, you wonder what you did wrong, and you go over the skill/station over and over and over, study the material, study the procedures, study the things that are bad, the things that are right. Telling a person what they do wrong doesn't help you in the long run. Sure, it would be nice to know, but not being told, forces the person to go back and study everything so when they are in the real world, they don't kill someone.
by the way... it's pretty hard to fail a station if you pay attention and know what you are doing.