...maybe your model is to "thin". I experienced trouble getting stuck in some of my models when the face i walked on/into was to close to an opposite face whose normals pointed in the adjacent direction. Think of it like a sandwich where you walk on the upper slice while the lower slice is tight (not enough ham!!
) to it- That gives you a good chance of FPSC-glue. In "segment units" (100 units=1 segment width/height) i now try to stay above 5 units with faces opposing each other.
Hope to help my fellow modellers with that small bit of information....
I am Jacks complete lack of surprise.