We brush regularly and also use a "hairball formula" dry food. Our kitty was also getting a small amount of canned food daily. He would throw up occasionally on that regimen. A couple of months ago I tried a new canned food that kitty absolutely LOVED. He went crazy for it. Problem was, over time, he started throwing up daily. We decided to eliminate the canned food altogether (didn't even go back to the original) and he has not thrown up for the past two weeks.
I don't know if your kitty gets any canned food, but if so, you could try dropping it altogether and see what happens. She won't be happy about it though!