I didn't get very high when I smoked, either, and I attributed it to the fact that I wasn't smoking enough, but my sister, who smoked the same amount, was flying. It may just be a difference in your brain chemistry.

I read a depressing fact a couple of weeks ago about long-term cannabis use that I'll try and dig up. It said that the longer people smoke, the less they get the desired effects from cannabis. That is, the highs get lower, and the other thing it said was that, after years of smoking, the negative side-effects (like coughing or irritation or maybe even forgetfulness) get more noticeable.

Maybe take a nice, long, three-month break and see if that helps. If it doesn't, try hash or something stronger.