Pain medication isn't working, why?

I have an uncle that had knee surgery a couple weeks ago. He was given Oxycodone for pain medication, at first it was helping but then it stopped working. The doctors took him off his crutches and the pain came back but this time the medication wasn’t working. In the beginning of the week the pain was so bad he wasn’t sleeping at all so he went back to the doctor. They said that everything looked great and that there was nothing wrong. The doctor gave him Dilaudid however, that isn’t working. Has anyone experienced this before?