15+ dog keeping weight on??

Hello everyone! I am new, and in need of advice. I have done a lot of research and still cannot find the correct food for my two Seniors. I have a Dobbie and a Walker hound – both are 15 years old. I originally had them for three years on chicken soup for the senior dog but due to this food being incredibly difficult to find anymore I have had to change dog food. Since then I have tried Canidae Grain Free Pure (Dry), Castor and Pollux Natural Ultramix Grain-Free (Dry), Earthborn Holistic Primitive Natural (Dry), and Solid Gold Barking at the Moon (Dry). The Walker Hound (Forest) did not like the earthborn or solid gold, the wellness grain free was too rich for him (he got really gassy and he has had bloat before) and the others did not keep weight on the seniors. I am at my wits end, what was not too rich is not keeping weight on either dog. Any suggestions? Nothing is physically wrong with them, they check out with bloodwork and the yearly physical. I have upped the feed amount but the Forest doesn’t finish his food and Emma I mean 2 cups per feeding twice a day is a lot. Thanks in advance!
Red