You have a different problem that is most likely not MED's fault. You probably have a triangle in your face that is not obviously visible. For example, you could have two vertices that have two lines connected between them instead of one. As you lay out the skin it will be impossible for you to select this triangle of no width.

Another problem could be that you have two vertices in the exact same postion. This will also give you a poly that is virtually invisible.

Your best bet is to double check your mesh.