R.L.
I have had many many dogs over the last 2 decades, and it is possible that changing the food abruptly has caused this. The best way to introduce a new food, especially to an older dog is to begin mixing a cupful of the new food into the regular feed, and then gradually increase the amount over a period of a few weeks, hence allowing the body to adjust to a new formula/food.
Also, there are many diverse dog foods these days, beef bone marrow/meal should be one of the ingredients to help with a dog's digestion, especially for larger dogs.