Don't be surprised with new treats or foods if your rats approach it cautiously, take only a small nibble and appear to be rejecting it. They will likely go back later and eat it. This is how rats normally try foods they don't immediately recognize. Most will just take a small bite then if it doesn't harm them, go back and eat more later on. One of the many reasons it's so hard to kill off wild rats...they don't always rush in and eat much of an unfamilar food source.