Help me please.

my mum has two whites tree frogs. I am normally in charge of feeding them every night. I do not always put fresh locusts in but there are always at least 4 in the tank. Recently I have noticed that one of the frogs has become incredibly skinny and won't eat anything. He tends to avoid locusts and bury himself into the ground, The other one is completely fine. I have read other forums but I can't find one really relating to this specific case. Please help me.