Why are my dogs ears getting so dirty all of a sudden?

We got a dog about 3 months ago and he had an ear infection. We took him to the Doctor and they cleaned his ears out and gave him some medicine. Then our other dog that has never had problems with her ears got an ear infection we took her to the vet and got some drops and all that good stuff. But now both of their ears are getting really waxy, but this time it is the other ear on both dogs. I never had a problem with our first dog, she never had an ear infection untill the new one came along. One thing I noticed is that I did start feeding a different food formula. I used to feed the Bluebuffalo Chicken formula and I switched to Salmon and Potato formula. We have been cleaning their ears out on a regular basis but they are still getting waxy, Not irritated, just waxy, and they let us clean them so I know they aren't hurting. What could be causing this.