Im a boy and have medium lengthh hair that i straighten and this happens to me when it gets longer! Straightening works by breaking hydrogens in your hair (water contains hydrogens thats why hair goes curly in rain) and this is just a guess but maybe you dont have enough hydrogens in your hair to break. Maybe its your shampoo or whatev. try different ones!
There's no point just clamping the straighteners onto wavy hair and holding them there - that will just lock the wave in. You need to pull the straighteners through the hair slowly and firmly, so that the hair that's between the blades is straight when it goes in - and make sure you keep the pressure even along the full length of your hair, otherwise you'll end up with a crimped look.
I think it's your ends, Try heat protectants they don't only make sure you don't had slit ends(which don't respond to heat) they also lock in the heat. So you;ll have to straighten less often, when you do it'll actually straighten, and it won't turn wavy in just an hour. Hope this helps. Also hairspray is an alternative, but i mean you have to get a really nice one for that to work(you spray after you're done straighting).
Sorry here are some I'm not sure where they ship though :P
^the last one is my favorite as long with the Matrix Heat Protector, and Tresemme(:
try using a heat protection spray, and then hair spray after you've done it. they don't sound like very good straightners if they take ten mins to heat up! get some ghds they take 30 secs!
Well maybe you live in a humid place also it could be the straightener that its not that good. Try using straightening products like shampoo & conditioner & oil works well to straighten hair. Otherwise, i don't know what's happening to your hair if it is like you say its really weird because you don't say you have really curly hair. I really don't know.
Sorry & Good Luck
Is it possible that your straightener isn't getting hot enough? I've had that problem with curling irons before.
do you live somewhere humid? my hair is like that and whenever it is humid out it just doesnt straighten that well. try some frizz and heat controll shampoos and hair conditioners