1. you can't convert a skin into a model. these are two different things
2. if you import a model, you also have to import a skin to apply to the model. the model is the "flesh" of the character/unit/hero/building and the skin is the colouring of the model (thus its name).
now try using a different skin for the same model (there are plenty of those here at wc3search). if it doesnt work and you're sure you did everything right, then i suggest to reinstall warcraft, because in that case your editor would be broken/corrupted or something like that.