If she is on frontline and scratching a lot still I would check for live fleas first. If you can't find any she may be having an allergy. From what Ive read here a good food for allergies is Natural Balance (the one with duck in it). Dish soap might dry out her skin and make her more itchy.
Some of the foods people here suggest is Canidae, Natural Balance, Wellness, Solid Gold. Im pretty sure you can get these at feed stores and I know petco sells Natural balance and solid gold. I think Iams isn't considered a high enough quality food for yorkies. Try doing a little research on those foods and see what you think.
Some of the foods websites can tell you where to get them near where you live.