I don't mean to insult your intelligence but bearing in mind your newness to MX bikes, are you letting it rev out before changing gear? A good way to find the power band is to get up into third gear at fairly low revs, make sure you are hanging on, and open the throttle all the way and wait to get to top revs. That way should leave you in no doubt as to where the power relly kicks in assuming everything is working properly. I had a '99 RM250 and the power came in with quite a bang in about mid-range, but then flattened off pretty quick on top. To check the power valve operation you could take off the right side power valve cover and then blip the throttle, you should see the actuating arm working the main power valve shaft. If you still suspect that the valves are not opening properly suspect the little allen headed grub screw secures the main shaft to the main valve. To check this remove the main power valve cover on the front of the cylinder. I had mine fall out once, it was very confusing for a while!