There is only human error haha.
A false positive is easily explained by contamination from a pipette or syringe. Maybe some liquid splashed up onto the non-disposable part of the pipette, and maybe the same thing happened again causing a free amine solution to get into the test vial.
A false negative could occur by improper heating, bad reagent mixture, etc.
I would try the K-test with a free amine in low concentration. Then you will know that you for sure added an amine, and how long it will take for the test to show a positive. If it does not show a positive, then you know it is a false negative, and you need to either heat the mix for longer or remake the test solutions.
Does that make sense?