A flaw I see in most pitchers than can’t get over the hump is being front-hip dominant. It can be a mobility constraint or a motor skills flaw but in most cases they had a coach that wanted them to get down the mound to a certain distance (usually at least 100% of height). I’ve said in past posts, the stride doesn’t matter if you can’t stabilize it. Stabilizing meaning accepting ground reaction forces through the front leg.
A great cue, I saw from Matt Blake talked about having pitchers trying to feel their landing. “Stride” for force into the ground. This will not only help eliminate reaching in the stride but should help keep tension in the hips, and in turn eliminate a disconnect between the lower and upper half. Basically, if you can’t stabilize the hips, the hips can’t rotate and flex as efficiently. You’ll notice the priority in the stride will no longer be to get to a certain spot but to land hard which will in turn help rotate hard, which is a key to velocity.