I am a fentanyl patch but my work drug tests come up negative. How can this be?

Fentanyl is. Unique pharmacolocig compund that may cover narcotic/opiod receptors but it and its metabolites may not be picked p in generic screens for morhine, heroin, Methadone or cannabinoids. There are other newer pain products Nucynta (tapentadol) that also are relatively potent analgesics that do not turn screens positive.
Using it properly? Assuming you are administering the patch properly, and are experiencing pain relief, it may be that the drug tests are not testing for fentanyl, but other drugs. It may be that you are new to the patch, and somehow aren't using it properly. Bring the patch with you for your next appointment with your treating physician, and show him or her how you put it on.