I get the feeling that this was the whole idea with captain shota; they billed him as this amazing genius who rose so far, so fast and gained a bankai in such a short amount of time, so everybody assumed he had this vast potential. That he would one day become something even greater.
But every fight he hits the wall, showing that you can have all the supposed potential in the world, but at the end of the day it doesn't mean a damn thing if you don't do anything serious with it between fights. He's sort of like Frieza in a sense, where he was very talented without really trying, all of this power and ability came so easily to the point where he didn't feel the need to improve that much. But when it came time to do so, he rushed and half assed it, only to be shown that again, it doesn't matter how much potential you have doesn't mean a damn thing if you don't do anything serious with it.
We see that with his bankai, where it was so incomplete it had a a built in time limit, yet he used it anyway, and when he was beaten he did the bare minimum to improve it to some form of completeness, only to be beaten again. He coasts on the bare minimum of improvement for as long as possible, and when he gets beaten he does the least amount of training until he feels like he can just go with it, then gets beaten again.