Being from a store, it may need a dose of Panacure for internal worms or Flagyl for flagellate parasites. These will increase appetite and clean out any bad buggers inside.

At that weight small adult mice would be ideal or rat weans at 4-6 weeks old.

Are all of you enclosure stats correct? Hot side 90/92, ambient temp 78/80, humidity 60-70%, hides, etc?

If everything is perfect and the problem persists much longer I would call around to find a vet near you with experience with snakes and take it for a check-up/deworming.