Some dogs will eat anything, so it's hard to tell if they're actually enjoying their food or just swallowing it because they're used to it. When changing foods, it's especially important to be vigilant -- you want to make sure he's eating enough, especially if you have another dog who tends to clean up food bowls when nobody's looking.
Observe him when he eats. Does he push the food around with his nose, as if he's looking for better stuff buried under? That's an obvious sign he's not enjoying his meal. Try offering a treat or a bit of wet food on top of his regular food. If he's suddenly very interested in the food, then it's the food -- and not lack of hunger -- that's causing the slow eating.
Check the food to make sure it looks good. If you buy large bags, there's always a chance it might get moldy or attract insects if you don't store it properly in an airtight container. If there's something wrong with the food, it might be that Rover is objecting to, rather than the food itself.
Check the food bag to see how much you're supposed to be feeding. Are you feeding too much? A lack of interest in food might be due to a dog not being hungry, rather than not liking his food. If you cut down on the amount you feed, and Doggie goes back to eating normally, the food might be acceptable for his taste.
Introduce new foods slowly. Dogs are creatures of habit and a sudden switch to a new type of food can cause many reactions. Maybe Doggie is suddenly refusing to eat, or maybe he's eating normally but his tummy is paying for it. When introducing new foods, mix with the old food at first. If Doggie is still eating normally, he might not mind the taste combination after all. Slowly decrease the amount of the old food and increase the amount of the new food until you've made a complete switch.