Princess Sparkle

Hey guys, looking for a little advice!

I got my first leopard gecko's last week...three of the little darlings and I love love love them!!! HOWEVER! To start with none of them would eat and now one of them is much smller than the others and it still refuses food. I have tried waxworms with/without sup. and hand feeding as well as seperating the poor little chap.

I would take it back to the shop for help, but the guy that sold them to me is a nutter!!!!

Any advice?? I'd be so sad if it died and I've no idea what to do