Mistakes will cause some of the arc to be left out of polygons. These are called orphans. If you have any orphans, you will need to go back into your arc file and correct the problems. This file will contain the orphan arcs. Turning on the nodes using the 'Nodes' choice on the lower menu will help in seeing the direction of the arcs and determining which are the right and left sides.
Green nodes are start nodes and red nodes are end nodes.