What is in dog food that causes my dog to be sick?

[jcn]

Ever since I have had my Chihuahua (about 4 years) she has had a major problem with eating dog food. When I first got her I didn't know too much about dog foods and thought I was doing well by buying her Nutro or Iams. She would frequently be sick- regurgitating food and water, loose stools, etc. She was never sickly like loosing weight or anything but you could tell that she didn't feel well. I spent hundreds, maybe even thousands of dollars at the vet and they could never find out what was wrong with her. I started to learn about dog foods and started feeding her higher quality kibbles, I tried things like Wellness, Natural Balance, Eagle Pack, Solid Gold, etc with the same results. Eventually I started cooking her food. Her problems stopped entirely. I would like to know though what ingredient in the dog food causes her to be sick? I don't believe that it is the protein source as in her homecooked foods I have given her chicken, turkey, beef, fish and even well cooked pork with no problem. Wheat and oats aren't the issue as I have also included those in her homecooked meals. She doesn't seem to have a problem with any veggies either including corn and potatoes. She is thriving on her homecooked diet and her vet agrees that this is best for her so I don't necessarily want to change it, I'm just curious what could it be about the dog food that causes her to always be sick when she eats it? None of my other dogs have had a problem with any of the foods that she does.