My parent's adult female sheltie has always been picky, but recently she completely quit eating her food. She was eating several premium brands, but after about a week on them, she would get tired of it and quit eating. She has become very thin but the vet can't find anything wrong with her. She is just very picky. Sadly, they've had to resort to feeding her Beneful since this is the only food she will eat on a consistent basis. Any ideas on how to make better food more appealing to her? They've tried almost every brand out there but sadly she still choses Beneful.