Why wont my bearded dragon eat?

i have two 6month old bearded dragons. one (lizzie) of them is happy and healthy and is fine, the other (rosie) is skinny, weak, and much smaller than my other one. they are both about the same age, and eat the same diet. for the last 2 weeks rosie hasnt been eating the crickets i give her, but she will eat the greens.. i recently moved them into a 70 gallon tank and lizzie is taking nicely to it. i have also switch them to calcium sand. can someone please help??? im very worried about my beloved lizard!