Need help with my snake?

well i have 3 snakes and one i got from a friend because it wasn't eating i got it to eat but that was like 6 months ago and i have been trying to get it to eat ever since my other 2 snakes have no problem eating yet this one won't i've tried everything i could think of feeding it at night feeding rats mice and thawed ones. i worried for it because my other ones are so big and it seems to be shrinking they are all ball pythons if that helps