The one thing that will stop Makefaces dead in its tracks is if there are any stubs of lines inside the overall outline shape. (Think of a simple case like a circular outline with a tiny stub of a "spoke" heading from one of the nodes on the edge towards the centre). If you are sure the outline is compltely closed and planar, then that's the most likely reason. You may need to fill it in by progressively drawing lines across the shape between obvious points...then delete them once the complete shape is filled.