You say that she had monthly bouts of the runs and I am wondering if you found the cause of this and if you had a diagnosis from the vet? Is she still having bowel problems or has this completely cleared up? I just wanted to rule out any digestive issues. One of my dogs used to go off her food but she had colitis and that was a significant factor. If you feel that her condition warrants it, have her examined by the vet. If she has pain the vet will be able to tell when he examines her.
What is her oral health like? Does she have any tartar, bad teeth or sore gums? If so, see the vet for advice.
The other issue is overfeeding. Is it possible that you are giving too much? Do you weigh her food to make sure that she gets the correct amount each day? If not, buy a digital scale and weigh all her food. Obviously don't give treats as these will take the edge off her appetite.
You don't say whether you are giving dry or wet Chappie. If it is the former, try adding warm water. It makes the food smellier and aroma is what attracts dogs so they enjoy it more. The Chappie dry review on the Dog Food Directory is here.
It contains a lot of cereal and your dog might just not find this appetising. Dogs need good quality, digestible meat protein.
Fussy eaters often do better with wet food but again, it is important to give the right amount so weighing is useful, at least at first. I understand why you would not want to change her food again but you will have to weigh up the pros and cons of this. We can help you to find something else. Remember that the better quality foods have a named meat source at the top of the list of ingredients.