Byakuya & Kenpachi power up?

Ok so Bleach fans know that Ichigo barely beat Kenpachi & Byakuya right? For the end of Ichigo's Kenpachi fight it was his spiritual pressure that was between assistant and captain class, Zangetsu's spiritual pressure (and Zangetsu is an unsually strong zanpakuto) and Ichigo's inner Hollow told Zangetsu he'd let Ichigo use his power, and the inner Hollow makes Ichigo far far stronger. Three extremely powerful spritual pressures against Kenpachis and it ended in a tie (everyone says that since Kenpachi said Ichigo won that Kenpachi lost). Then during the fight with Byakuya, Ichigo could no longer move and his Inner Hollow had to take over, and because of the interruption Ichigo won. So why is it in the Hueco Mundo arc Ichigo had a hard time fighting with Grimmjow (Espada 6) even with his Hollow mask on while Byakuya massacres Zommari (Espada 7) who's a bit weaker than Grimmjow, and Kenpachi completely dominates Nnoitra (Espada 5) who (from what I could tell) was on a whole other level from Grimmjow? Plus even before Byakuya joins in the fight against Yammy (Espada 0), Kenpachi was tearing Yammy apart. So if Ichigo when he was weaker managed to beat them both (even if by luck) why the hell are they over powering the other Espada? Seriously, why weren't they this strong during the fighting in Soul Society?