Do you use heat protection spray? My hair was like this for years, still is to a degree but in lockdown it’s become much better, I switched shampoos (I use a combo of the garnier hair food mask and Aussie shampoo) and stopped using my hair straighteners (and hairdryer where possible!)… if I do use my hair straighteners I mist my hair in heat protection spray, I find it’s become much less greasy and much thicker/better condition too!
Get rid of some product build-up in your hair by using a clarifying shampoo (be careful if you’re hair is coloured as it may strip the dye), give your straighteners a clean as well as your hair brushes.
A few others have said it but I think this could be the wrong products, particularly conditioner. Perhaps you're using too much or one that isn't compatible with your hair?
Is your hair fine? Mine is too & this can happen to me, just looks lank at the roots. I’ve found that rotating the straightener a little bit gives a little bit of lift at the root & helps with that. Also a decent dry shampoo ( depends on your budget, but I’ve found the Living Proof one is the best). Good luck x
My hair is thick and cut in a graduated bob. It’s only the front that looks greasy after straightening. I’ve tried loads of different shampoos even expensive ones.
When this happens to me I clean my straighteners, use head and shoulders to really clean my hair before using my normal shampoo or add lemon juice to my shampoo to help remove the build up of products, give it a blast of dry shampoo and hope and pray to the hair gods to make my hair nice again!