I had the same problem. My boxer came eating Purina Puppy Chow and I knew I had to change that right away. I changed to Nutro Puppy over a two week period and it was terrible. It was like water and she had no control over her bowels. This made training impossible. We had to switch back to the Purina (which I think is a terrible food) to get her back on track. I am now slowly changing food again (1/4 cup of new the first week, 1/2 cup of the new the second, etc..). So far so good, but we are only on week one. I also went to Natural Balance Ultra Premium and stayed away from Nutro. I feel for you. When you make the change be sure to do it slowly.
Corky, fawn, natural ears (F) - 6-15-97; went to the Rainbow Bridge 1-26-2004
Xena, fawn, natural ears (F) - 3-29-02
Lexi, flashy fawn, natural ears (F) - 12-11-2003