Thick, flaking skin in instep. What may this be?

I have a skin problem on my right foot . It is in the instep area. the skin there grows thick and then seperates. by this I mean it sheds off. sometimes it goes deep and becomes very irritating. I went to a dermatologist for this and was told he had no idea but did give me a couple of meds to try. One orally and one tipically to which neither worked. He said we may never know what this is. I have had it since 1994 and it is only on the right foot. It never completely clears up or goes away and the area is getting larger. Any ideas as to what this may be and what to do for it?