@Painful One gave a really great answer here. I'm basically gonna echo a lot of the points she has already made.
There is absolutely a "sweet spot" with Opioids. Lower dosages bring more euphoria, analgesia; essentially all of the effects that a person wants from Opioids. Past certain point, when the dosage is raised, the experience really changes.
I tell a story often of when I was in Cambodia as a teacher. I could basically afford all of the Heroin I wanted and still have money for everything else. I had always assumed that infinite Heroin = infinite happiness. That is how I understood the drug.
When I got to Cambodia and I had nothing tying me down, I eventually got to a point where I was inecting up to 4 grams of relatively pure Heroin per day. It just started making me feel numb, shitty, empty, hollow. The Heroin would make me more sedated, more itchy, but it didn't make me feel better.
I remember I got to a point where one day I had done my two shots (it too at least two syringes to appropriately contain all of that sludge) and I felt like I was imprisoned in my own body. I couldn't feel anything, good or bad. I screamed as loud as I could and banged my head into my desk as hard as I could. I couldn't feel anything anymore. My mind kept telling me that more Heroin would make me feel better, but it didn't.
I eventually decided to bring my dose down on my own. I wanted the good feelings back. I was tired of feeling numb.
I eventually found that I felt best at 1.2g-1.5g per day, but I might have even felt better on less. I'm not sure. However, once I got back down to htat level, my injections gave me relief. They gave me what I was looking for. It was an interesting experience. I never would have expected all of that Heroin not to make me feel better.
My suggestion would be, if you're not feeling enough from your current dose, I would try lowering it. They key to these issues is not in taking more, it is in reducing the dosage and frequency and trying again. The more you take, the more your returns will diminish.