This is the way I see soul slayers: every soul slayer has Mode #1 and Mode #2. Obviously, Mode #1 is the one that everybody starts at. Mode #2 is the one where you can say *action* *soul slayer name* and make it become stronger in a way.
Right now, Ichigo's soul slayer is stuck on Mode #2. Kenpachi's is stuck in Mode #1. That's the way I see it.