Fat cat/dandruff

Earlier this year I started transitioning to almost all organic foods, eliminated toxic cleaners, etc., and then realized I needed to change my cats food too from mainstream to more healthy. I think I might need to take it up a notch though.

Currently they eat Pet Promise, but one of them is overweight and has bad dandruff. Wondering if it's the food. Could it be something else? The other two are in great shape. Any ideas? Much thanks.