What are your water parameters? What about pH? How long have you had them? How old is your tank and is it cycled? What size tank? What other inhabitants and how many of each?
Try feeding a shelled pea incase it's swim bladder disorder. It could be sick. Neons have weakened immune systems due to mass in-breeding so they can get all sorts of nasty illnesses rather easily if not kept in an ideal environment. If you see it becoming sluggish or acting lethargic or not eating, isolate it from the other fish and treat according to any new symptoms.
