How can i get my dog to gain weight?

Hi, i have an 8 year old boxer mix. She was orginally a goo 45 pounds and very healthy and muscular, then she started rapidly losing weight, she go t down to 19 pounds and we took her to the vet and all her blood work was good, they did x-rays and she looked perfectly healthy and all the vet could come up with is that she has irritable bowel syndrome, so they put her on prednisone, famotidine, and tylan powder. it has been maybe 5 or 6 months and i took her to the vet and she weighed 23 pounds so she gained some weight, but you can still see her ribs and her backbone, she is a happy little girl you cant tell shes sick except she acts like shes starving all the time and she gets fed twice a day. so i didnt know if anyone had any ways i could help her gain weight, please and thankyou