It can take them 10 days to two weeks to fully settle into a cage and familiarize themselves and it can take a couple of days for them to decide where to nest and hoard. I am sure they have taken some food and you haven't noticed. Maybe add a few healthy treats around the cage and see if they disappear. Eg a few pumpkin seeds, half a walnut etc.
At least he or she likes their wheel! If you can post a photo we can tell you if they are male or female
Males have a different shaped back end.