What Causes False Negative Xanax And Hydrocodone In Urine Test?

I have Rheumatoid Arthritis and chronic pain (20 years). I have been on hydrocodone, and occasionally on oxy and Xanax for 10-15 years. Right now I am on 5 10/325 hydro per day, and 1 to 2 .5 mg Xanax per day. I take urine tests almost every time I go to this pain clinic. They keep saying that my Xanax does not show up in my urine tests, that a couple of times oxy didn t show up (I didn t always take it because I didn t like the effects) and now they are saying my hydro isn t showing up at the right level for the amount I am taking. Please help! This is crazy. I absolutely take my medications, other than the oxy which I am not longer on, and I feel like I am being treated like a criminal. I have done nothing wrong. I am not selling, hording or giving these meds away. I need them! I am now on probation and this clinic is threatening to dismiss me. What recourse do I have?