The humeral elevation is too high!!!1!!!1!!Though this is clearly fanciful, I remember reading some online speculation on whether the hatchlings of medium-sized dromies could have had some gliding ability.
Albertonykus: how intriguing. How disappointing though to have to lose one's ability to 'fly' as one matures. But such is life. Bah.
There's a real life example of that, in fact (and it was what the discussion was initially about). Juvenile megapodes (eg: brush turkeys) are better at flying and WAIR than the adults, which tend to just run instead. Young megapodes have also been known to make sea crossings, while adults rarely fly very far.
Extraordinary. There I was, trying to draw an analogy with some of us less happy souls, but there turns out to be a real life bird example. This is the first I've heard of this, so shameful is my ignorance.