i allready writte about this some time ago in one of treaths.
my main point was that more power ichigo get more he open path to his hollow.
-right after training with urahara he have sealed hollow, but is still there.
Bleach 64 page 12-13 | One Manga
-he learn shi-kai hollow made mask in fight against renji in order to protect him (so seal was littre broken, by using skill of his shi-kai)
Bleach 100 page 04 | One Manga
-metaralizacion (when he was on edge of death in fight against zaraki zangensu transported him in inner world, where he fight against his hollow without knowing it.
and what they wanted to taught him was that he should learn more about zangensu in order to breake seal, in other worlds they wanted him to learn ban-kai)
Bleach 111 page 08 | One Manga
Bleach 111 page 09 | One Manga
Bleach 116 page 18 | One Manga
-and finaly ban-kai (zangensu can finaly show his true form and 100% of his power.
but at the same time seal is broken and hollow have free path to ichigo's soul.
in fight against byakuya hollow first time took controle over ichigos body)
Bleach 165 page 19 | One Manga
and we know that than he was comming closer and closer to ichigo and forced him to go trought vizard training.
Bleach 193 page 13 | One Manga
Bleach 212 page 05 | One Manga
Bleach 196 page 10 | One Manga
Bleach 216 page 12-13 | One Manga