Pot definitely potentiates opiates (as well as benzo's, tranq's, some psychs and MDMA & E) but the issue is, is that your tolerance is either too high or you aren't doing enough. Which go hand in hand. I don't know a single person who doesn't nod on opiates. Any opiate. You just have to do enough. And I'm sure once you find that magical Mg amount, you'll really start enjoying opiates more.
Nodding is your bodies defense so you don't do anymore and die. If you aren't nodding then you definitely are not doing enough.
I'd like to note though, that some opiates are more sedating than others when reacting with your body, which opiate is your drug of choice?