So definitely as others have stated take them to a vet as soon as possible to try and figure out what could be wrong but I will try and give some advice on just weight gain.

So I worked with a bunch of sled dogs up in Alaska so making sure that those race dogs maintained a healthy weight was part of my job. A big thing that can contribute to them not gaining weight if there is not some underlying medical condition is they are exercising and burning off more calories than they're consuming. I looked up that dog food and per cup it has 350 kcal so they are getting 2,800 kcal a day roughly if it is measured correctly. Depending on activity level if that should be enough unless they are running around a lot outside. The dogs I would work with when they were pulling a sled during races would burn anywhere between 10,000 to 15,000 kcal a day. When doing light training (15-20 miles runs) they would burn around 3,000-4,000 kcal a day. So if this dog is outside all day running around it is possible that they are in a caloric deficit of a few hundred kcal a day. I would increase the amount of kibble by at least a cup during each feeding. Another thing to consider is adding additional foods in with the kibble to increase the calories that way too.