If a large shape won't fill, draw a line from one corner to another. This often forces SketchUp to recognize the plane.
If this doesn't work, you likely have a gap or a non-coplanar line. Check your endpoints to ensure they are truly connected. sketchup make face
Next time you see that empty wireframe, don't panic. Just trace, script, or plugin your way to a solid surface! If a large shape won't fill, draw a