Although Logia user CAN turn their body into their respective elements (notice the capitalized "can"), it isn't automated. Powerful Logia users train their reflexes to the point where it seems like it is automatic, but in fact it's just a very quick reaction.
What happened with Smoker was that it was so quick and unexpected that he wasn't even able to make himself intangible. And although he is a badass and very strong, he was nowhere near an Admiral's level, and so his reaction time wasn't as perfected. That's why Smoker didn't react to a slingshot Luffy but Kizaru was able to react to a bullet shot from behind.
In Caribou's case, it's not that he couldn't react, it's more because in that situation (He was completely alone in a foreign ship surrounded by powerful opponents), it was more profitable if he were to hide his powers and show defeat, so that he could plan his escape route when no one was expecting (Which he did, albeit stupidly got himself captured again).
It appears I was only partially right.
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I was right in the sense that you can't train yourself to awaken it, nor can you train it to grow. The power grows stronger as the user grows stronger. So you were right, the power did grow, not because Luffy trained it, but because Luffy himself grew stronger. Likewise, as I said, the actual training he did relating to this kind of Haki was only to Control it, not to enhance it.
