I have a Heinz 57, but we all love him. The previous owner said his skin problem was hereditary. we use ivomec?

When I first moved in his previous owner had more or less neglected him. He had hardly any hair left at all. We treated him once a day 1ML of Ivomec daily. It cleared up and he grew his hair back in about 1 1/2 months. This was 2 years ago since we had only needed to treat him once for maybe 2 weeks and It cleared up again. Now he has small brownish black spots on his back and sides, his skin is flaking as well. The Ivomec is not responding this time. She is not itching at all. Anyone have some suggestions ??