What did you find out from the Vet about your dog?
Incidently, be sure you always EMPTY the bin completely before adding the next bag of food. Or at least take out the kibble at the bottom of the bin, poor in the new food, then put the "bottom dregs" back on top of the new food. If you never empty the food from the bottom, it can spoil, so you may simply be feeding them spoiled food from the bottom of the bin that has been in there for over a year.
But also... a number of people are reporting problems while feeding Iams dry. I hesitate to say the Iams CAUSED the problems, because it's difficult to prove cause/effect, but it's just a caution that people are reporting health problems who happen to feed Iams dry (myself included).