Focus on punching fast, not hard. When you just try to punch really hard you'll end up tensing up your muscles and it'll come out slower than it should. If you, instead, focus on punching fast, fluidly and with good technique then it should end up being more powerful regardless. A nice crisp *SMACK* when you hit the bag or pads means you're hitting hard, not how far the bag moves.
Also, your hand, when you're bringing the punch forward, should be corkscrewing in to your target. Don't set it so that your knuckles are paralell to the ground then punch it forward.
Outside of that, I dunno. It'd be easier to tell if I could see a video of you throwing some punches.
"I don't want them to like me, because when I'm in the ring, punches fly to the face. It is not a tenderness time. I am not falling in love. And he knows that when the fight starts I am kicking faces."
- Wanderlei Silva