OP, you should probably be aware that some folks, myself included, experience certain Opioids differently than the majority of the population. I feel, based upon my limited research, that this is probably related to a difference in our bodies' metabolic processes and respective enzyme activities. For instance, I'm not unaffected by Hydrocodone or Oxycodone, but I would require at least 100mg of either to experience a decent high. This is enough to overdose a lot of people. The same goes for Codeine. I've take grams at a time of CWE material, with little effect.
Other Opioids, specifically Morphine, Heroin and Hydromorphone seem to effect me normally, albeit with a higher tolerance than most people that I know. You could be experiencing the same phenomenon. You might want to give some of the stronger Opioids a try to see if they effect you differently. There's really no practical difference from one Opioid to another in terms of potency; it's all relative.