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Black and Whtie isn't a reboot to the series, they even mentioned previous series' when Ash's Charizard return. It's all a connected story. However, I speculate that the reason for Ash being such a terrible trainer in the BW series is pretty much him coping after his defeat in the Sinnoh League and pretty much not caring much, you can see this change go in the beginning of the XY series. Even in the XY series Ash remained clumsy for a few first episodes, this later changed, he has matured and look at him now. A badass team that's greater than his Sinnoh Team.
