Two possibilities-she is bored OR something is missing in her diet. I would put a “toy” in her stall. Hang a large ball or a gallon jug with something to add a little weight to it (I put pieces of carrots in my gelding’s and a small opening large enough for the carrot piece to show through, but then he has to work it out so that when he plays with it, sometimes he gets a “treat”-it encourages him to play with it.) I would consult your vet regarding feed. As a LAST resort, you can use an anti-cribbing device on her halter…it is a muzzle-type device through which she can drink, but can’t graze or chew.