Why do some pills hit hours after ingested?

I'm prescribed oxycodone for pain, the first time i took it it did not effect me, so i waited 3 hours and took another does, still no effect so i wait 2 more hours and rake another dose and there in 20 minutes i start to almost overdose or feel too much of it as if all of the started reacting at the same time. Now i havent talked to my doctor about this problem yet, but is there any theories or suggestions for now?