Please Help! New Water Monitor Problems!

Hello all, about 2 days ago, I received 2 baby asian water monitors from an online reptile shop. One of the 2 looks great, is plump, active and chomps down on the food I provide. The other one on the other hand, looks very skinny, is very lazy and not so energetic and will not accept any food I offer which includes crickets, pinky mice, eggs, etc... I have reached out to the store I bought from numerous times via email and phone earlier today. All that they tell me is that the animal is either stressed or sick(well DUH!). They also blame my temperatures and humidity levels. I am not knew to the reptile hobby, my humidity and temp levels are high and where recommended for this species. One of the 2 looks great, the other one certainly didn't get this skinny by not eating crickets these past 2 days. There are hides, etc and the animal does not appear stressed, but rather sick. Tomorrow I will be posting pictures to this post, but for now it is bothering me and I am very upset. Can anyone help me with any tips or solutions, things I can try? I am thinking the poor lizard has parasites but am not certain. I have made a vet appointment but any ideas before then would be great, thank you all!