I was grooming one of my hens today. When I got to her vent I noticed she had a bunch of mites running around there. But every week since I got my flock (8 weeks ago) I've given them a dust bath mixed with wood ash, DE, and chinchilla dust (friend had a heap of it left after her chinchilla passed away so I thought it would be useful). Yet she and a few others have mites. I change the hay weekly. Should I change it more often? Am I doing something wrong?