Okay - let's us know if the primer bulb change out helps, but it doesn't sound like that's your problem.
Yes, try new gas and new pump up bulb in a portable tank. It's always good to have one around anyway for testing.
I assume you mean a 21" PITCH on your prop - you can't put a 21" diameter prop on a 225. Not sure what size boat, beam and weight you have, but a 21" pitch sounds pretty steep for most applications, unless you have a really small/light boat. I have a 25 ft C-Hawk with a 9'6" beam. Loaded it weighs about 5500 lbs. On my old Johnson 200 Sea Pro, the best prop was a 15 X 15. With the Honda 225 the best prop seems to be a 14.25 X 15. I get right at 40 mph at 6000 rpm on flat water.
You likely need to experiment with different prop sizes and pitches. If you have a dealer nearby, they often have a bunch of used props laying around. Ask to borrow a couple of different sizes and pitches and test them. The important thing is to make sure your WOT is right around 6000 RPM.
But, having said all of that, no matter what prop you have, you should not be bogging down at mid-range. From your description, it sounds like your V-Tec is not kicking in properly, but that should give a fault code. However, before going down that path. Play with some prop changes.