Board Thread:General Discussion/@comment-4957813-20131130185522/@comment-5553481-20140604194759

Kenpachi had placed "blocks" on his real power after he fought against Unohana as a child, until he finally managed to get through all those "blocks" could he hear the name of his zanpakuto, that's what happened in the second fight against Unohana.

Ichigo had no control over his reiatsu because "Quincy" Zangetsu was constantly trying to suppress his Shinigami+Hollow powers, but they were fighting back, that's why he couldn't control his reiatsu.