Writing in third person

I have a little over 150 books on my shelf. They are the middle grade and young adult books I have read in the last couple of years. Out of all of them, only 1/3 are written in third person.
Not surprising. Most of the writers I know chose first person as their choice POV. At the Big Sur Writing Workshop, most of the participants had first person stories. and the list goes on.
I don't think it's easier to write first person. Your character is right out there in the open, nothing to hide behind, so she better be appealing. With third person, there's a slight buffer. I chose third person to write my story, Greenwood Girls. Mind you, I tried first person once but it was suffocating. My protagonist was everywhere. I felt I needed distance. Third person was a perfect solution. GG's is about a athletic boarding school, like today's national training centers, but this school has secrets. It's another character in the book, and in many ways acts like the antagonist.
So why am I struggling?
Mostly, I have it down. The setting and the cast of characters are both good. But I still need work on my main character. I've had many critiques and most say the same thing. Or at least I think I hear the same thing; great concept but they want to see Dree a bit more.
It's not easy. Achieving the prefect balance between interior monologue (that is actually useful to the story) and dialog/action is tough. I refer to books to help me with this; Chicks with Sticks is the latest 3rd person book that I'm reading. Olive's Ocean and The Callahan Cousins are two others that I refer to. The Sisterhood of the Travelling pants is another. I look for how the author balances the quiet spaces between dialogue and action.

Editor Cheryl Klein often speaks about the art of writing in her blog Brooklyn Arden. Last week, she wrote this:
Also something I was thinking of reading a couple recent submissions: If your manuscript is in third-person limited POV, take one chapter, cut out all of the internal monologue, and stick it in a drawer for a week. Then go back and see how much of that monologue really absolutely HAS to be in the scene for said scene to make sense. The reason I say this is because internal monologue very, very easily slides into redundancy or telling, and it can also very easily slow up a scene (especially a dialogue scene, if the narrator thinks something after every line).

Good advice. I try most everything writers, editors, writing friends request. I still look for tips on hope to make my third person voice stronger. Any other advice out there?