User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-3146930-20130720154210/@comment-6068707-20130720204152

Remember that Riordan wrote his characters out to be equal but different.

I mean we can obviously see this with Hazel and Nico, as they have mentioned it in the series already. Hazel mentions that she and Nico represent both sides of the God of the Underworld, Nico (Death) and Hazel (Wealth).

So we can obviously say that Hazel will be limited with her powers over death if she has any. If not then it would be nothing. While Nico would be limited over wealth, this is why his Geokinesis is either equal to percy's or weaker, and substantically weaker than Hazel's.

Hazel can't control the earth above, because Pluto gave her the curse of ferrokinesis, indicating that her Geokinetic abilities would be tied to the riches under the earth, meaning anything about a certain limit she would have trouble touching.

As to the whole reason she isn't capable of using death abilities, that's riodan's mind.

For the Children of Zeus/Jupiter.

Thalia's fears have consumed her already. Note that Percy said that if he didn't get over his fear of suffocating, But Thalia seems to have let this overcome her, and that why the thought of being in the sky would scare her. Unless she is thinking something else while doing it. But I do remember that in the Titan's curse she was levitatting off the ground when fighting Percy.

So maybe she just needs to channel it in the same way Jason does. It just is different.

Im pretty sure that fear of heights isn't the cause of that phobia as well its more to do with the fear of falling. I mean I like roller coasters, but when I look down from a certain height I get scared. So Its most likely the latter.