Check the ingredients on the food label.
Corn (maize) in any form, wheat, soy (soya) and other ingredients can all cause an allergy that results in hair loss.
Get the cat on a canned diet, a food with no grains. Try Wellness.
There are also cats who develop allergies to the proteins in foods which they eat very frequently. In that case the solution would be to find good cat foods that contain none of the protein sources commonly found in cat foods such as chicken and turkey.
In that case you'd need to try a food like Natural Balance Duck and Pea.
I went through this with a cat of mine and it turned out to be an allergy to corn.
But don't listen to strangers on the Internet - talk with your vet about this and bring her in if the vet advises you do. She might still have some sort of skin condition that has nothing to do with a food allergy.