They may have been starved, intentionally or otherwise, by their former owners or the pet store. If you've not taken such an iguana to a reptile vet, do so. Forcing a sick and dehydrated iguana to eat hard to digest food could send it into shock, or worse. An iguana suffering from MBD or chronic starvation may not have a gut that is functioning very well. Much of its natural gut flora may have died off.
Spoon feed a teaspoonful of nonfat yogurt containing live cultures once a day for a couple of days.
This is very easy to digest and will help boost the gut flora. Emaciation and Force-Feeding Emaciated iguanas, or those severely weakened, from illness or starvation, should be started on an easy to digest high calorie food. Ensure comes in many flavors in several different formulas - select the regular formula, not high protein or low fat formulas. It can be offered by spoon, eye dropper or needle-less syringe. Gently pull down the dewlap and deposit a small amount in the mouth.