Itching in one particular place.

Hi, I have 2 Frenchies, a boy, George who is 18 months and a girl, Coco who is 2 yrs 4 months. We rescued Coco from a breeder who no longer wanted her 5 months ago. Coco was not very healthy when we got her but through various tests we have discovered her food allergies and she is doing so much better. One thing which hasn't stopped is that she scratches her vagina quite frequently with her paws. It can get quite sore.  The vets have ruled out mites, yeast infections and any other problems so I am at loss at what it could be.  Does anyone have any ideas?  Thanks.