My theory; you run out of horsepower. Solution; I would find someone with a dyno to see where that power curve is. If that sounds like a simplistic viewpoint, it is, kiss! It's good advice. Let's face it, I'm as dumb as anybody, if I was smart, I'd have money and I could afford real racecars.
Once you find out what the power curve says, tell me how it matches up to the RPM range you were racing with.
Rule of thumb; if you add a tooth to the axle, with the 4 cycle, in the 4.0 ratio range, plus or minus .5, approximately, you have to turn 100 more RPM to be going the same speed. Sometimes it's possible, that the power curve is going down so fast after peak, that putting on a lower gear, will not make you go faster. There's just not enough horsepower to go faster. "Gear bound" is just a shortcut way of saying the same thing. In my opinion, "gear bound" is just a term used by many to explain things that they can't explain. I put the term into the "self evident truth" category. Somehow they make sense out of something that cannot be made sense of. On most occasions, the person saying it, if they have the right credentials, everybody just nods and looks the other way. "Torque off the corners, horsepower down the straights" is one of my favorites in that category. LOL