If your dog is not gaining weight appropriately you need to make sure she is healthy before breeding her. Have her examined by your veterinarian to rule out causes of poor growth. It is very important that she does not have congenital diseases that can be passed onto her puppies. Intestinal parasites, nutritional deficiency, liver shunts, pancreatic insufficiency, and various enteropathies can prevent weight gain. Your veterinarian can work with you to get her healthy and in good body condition for breeding.