A couple of questins for you;
1. How old is the dog? If it is under a year and a half sometimes its hard to keep weight on a dog until they are full grown.
2. How active is the dog?
Also you might want to try a regular worming program, i know you said the vet said there is nothing wrong but sometimes they don’t know everything. You might also try a 26/18 dog food (18% fat and 26% protien). Always look to make sure corn in not the first ingredient you want to see something like chicken or a meat base. There are a lot of good dog foods out there but they are not cheap!!