I feed a mouse 10-15% of my BPs weight every 6-7 days. She was also underweight when I got her, but plumped up quickly. If you want to do 15% of the snakes weight every 5-7 days it should start putting on more weight. Get a good digital scale and record all feedings and weight gains so you know exactly how much weight is being gained. If it still doesn't plump up after feeding it properly, you may want to get it checked out for parasites.