This problem is generally casued by an error in the model (geometry). For example a flipped normal, open edges, ngon faces (more the 4 side), etc. Go through the model and make sure its 100% cleanly modelled. If you can't find anything see me and I take a look too.
hey adam! I had exactly the same problem. For some reason when I deleted half of the face and re-mirrored it, it worked. Try and see if it works for you :)