This is a very common problem in the pain management field. The semi-synthetics such as percocet do not show up because of the sensitivity of the test or rapid metabolism. This is a well known phenomenon regularly addressed in the medical literature and journals yet most physicians don't understand the pharmacokinetics of the drugs they prescribe and remain ignorant of the drug testing procedures.
“Family medicine physicians who order urine drug testing to monitor their patients on chronic opioid therapy are not proficient in their interpretation.” (Reisfield) These findings are supported by a later survey, 2010, of physicians at a medical conference determined that 88% of doctors didn't know that they may need to order a special test to accurately screen for oxycodone. This according to information provided at 163rd Annual Meeting of the American Psychiatric Association, New Orleans, May 22-26, 2010*.
Your result may be traced to the test or to your metabolism. Either way you will have great difficulty obtaining prescription from another doctor.
Reisfield, 2007
*Primary source: American Psychiatric Association
McCarberg BH (2011) A critical assessment of opioid treatment adherence using urine drug testing in chronic pain management. Postgrad Med. 2011 Nov;123(6):124-31
Family Practice, Table 3 2010