[Akiyoshi]

You're forgetting most of the bigger achievements Nanoha made in StrikerS was thanks mainly to the fact she was using a dangerous upgrade that forced her power to go beyond her limits literally puttin her safety and life on the line ...she sacrificed so much to fight those threats she actually received lasting consequences that took her some time to resolve.

Fate was also getting constantly worried about Nanoha's health back then, so it's implied that while Nanoha got a better grasp of her powers her body also got weaker.