Just like people different dogs will like and tolerate different foods. My 13 year old doesn't even chew her food - she swallows it whole. They all also eat birdseed that fell from the feeder, anything I accidentally drop on the floor (as soon as I go into the kitchen they stand around me staring at the floor...) and if they could get through the babygate, they'd eat the cat poop.
Talk to your vet, get some recommendations and go from there. Also don't change brands immediately - do it in parts - maybe add a few bites every meal or so for a week+. See what works for your doggie
