Hmm, I don’t know much about Rainbow High, but it could be that they’re made of different types of doll hair. For example, I recently worked with a G3 Clawdeen who had polypropylene hair; No matter what I did it was always kind of frizzy, while my Draculaura has saran hair, and it always lays nice and smooth with just a little bit of finger combing. So different hair types would be my guess.

hmm after a quick look at the database they all apparently have nylon hair, so hair type shouldn’t really be affecting it…
my hunch might be just that they were previously curled, meanwhile Tiara had mostly straight hair to begin.
feels like maybe attempting to straighten it with a straightener on the lowest setting might give some of the desired results?
however I am extremely inexperienced so please do take my comment with a grain of salt |ω・）(I’m only just starting my journey of doll hair styling)