I had the same problem as you and tried all kinds of treatments. My horse had deep Sulcus thrush. I finally found a product that worked! Its called NO THRUSH ! This stuff is awesome. It is a powder-not liquid. My horses thrush cleared up with in about 1 ½ weeks. I just made sure when applying it that I got it down in the sulcus all the way(his crack was really deep) I would gently and carefully separate his sulcus with a hoof pick and then insert the tip of the product and fill the crack with the powder. My horse was kept in a clean dry area at all times like your horse so I don’t know why he got it either. My only thought is my farrier always takes off a lot of frog and I have been reading a lot and according to how the mechanics of the frog is supposed to work it should barely be shorter than the heel so that it will contact the ground when the hoof flexes so that blood flow is stimulated. His were trimmed out so much that there is no way his frog would ever contact the ground to stimulate the blood flow so maybe this is why mine developed thrush-since his frog was not being properly stimulated?