User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-3146930-20130720154210/@comment-7742465-20130720190353

Maybe she's just never tried. Remember, Thalia and Jason are siblings, and both of them are children of Zeus/Jupiter. Jason can fly, but Thalia can't because she's never tried and because she's afraid of heights. Hazel has mostly focused on her cursed gems and her power over jewels, and has never had time to practice with other powers that children of Hades/Pluto can have.