From what I was told, most monks still have personal detachments. To truly reach enlightenment can take longer than their life.

LoK goes deeper into one of the most enlightened air nomads, which truly detached themself from life.

Aang reaching the avatar state is very different from of enlightenment though. Think of it as a boomerang, if Aang emotionally throws himself into the avatar state, it's likely that the metaphorical boomerang wasn't thrown very well. Aang nearly passed out when emotionally entering the avatar state, but he never triggered it willingly.

Once Aang learns to throw the metaphorical boomerang with no life attachments, the perfect throw would symbolize his attachments leaving him. He would have direct control before his initial throw, allowing him to accept it's leaving, and has the skill to bring it back, without it hitting him in the noggin.

So to add up: Aang hasn't taken his boomerang throwing practice seriously, and could only emotionally enter the boomerang state for a majority of the series.