Why percocet is not showing up in test results?

I am disabled with AV Necrosis in both shoulders and legs. My doctor prescribed percocet for my pain. I have to take 2 tablets (5-325 dosage ea) 2 times a day. I've been taking them religiously since they were prescribed (approx 3 years) according to my Doctor's directions. I had to take a urine test but had to take the test twice because the first test results came back in conclusive. The 2nd test results came back showing no percocet in my system. Can anyone tell me why this is happening?