klonopin screen negative

I have been useing klonopin for about 16 years, and still do. Well, I have a drug screen done and it shows neg. then another and it shows neg. also. using this medication daily and after taking it in front of my doctor I was sent for another screen about 2 1/2 hrs. later. This one came back, yep you guessed it NEG. What could be causing this not to show, why, how? I need help here, this med helps me daily, but will be cut if they do not find a solutiion for it not showing up. help