Is the "dose-ceiling" a real phenomenon or a myth? There's a lot of conjecture, I've found.
Some people are poor metabolizers of codeine, like myself, there is no amount of codeine that gives me anything more than an itchy nose - which could have just been placebo itself. I'm not saying that that is the OP's problem specifically, but it sounds like it. Codeine should be metabolized into morphine at a 10:1 rate. I don't think 40mgs of po morphine is a wicked hefty dose, but it should give anyone who is naive (or close) opiate effects. If I were trying to differentiate an opioid high from placebo, I would find some different substances to work with, as others have said.