Does she only have yeast in that one spot? Did the vet check between toes, under armpits, inside and around ears, neck?
Just like in humans, a yeast infection is usually on the inside - starts in small intestine. It's possible she has a local yeast infection. But if there is yeast elsewhere and this is not going away - you need a lot more than a cream. She'd need oral Diflucan x 6wks.
Either way, you should give her probiotics - this is the one I use:
Nature's Farmacy - Store - Product Details
You need to populate her sm. intestine asap w/ the right flora. Human yogurt is great, but you'd have to give her a ton of it for her to get enough of the species in her system.