Well, this happens because Warcraft III doesn't recognise it as a portait.
There is a special structure; if the main model is called "Warrior.mdx", the portait must be called "Warrior_Portrait.mdx".
If you have follows this structure, then make sure you don't use the portrait version on your model.
Finally, it might be a problem of the portrait model itself (for example a not-set-up camera); if it's a camera problem, use War3 Model Editor to make one.