Is Merrick dog food not right for my dogs?

I have my two dogs on Merrick Puppy plate right now. My lab is about 6 months old and my English Mastiff is a little over 10 weeks. They have had diarhea ever since they started this food and it has been at least two weeks. They are barely eating it too. Only when they are super hungry will they eat it. It seems like my Mastiff pup is losing weight too. I bought the food cause I heard good things about it, but am now thinking of switching back to Iams Large Breed puppy. What do you think. I can't handle the pooping everywhere anymore!