Sometimes there will be little lines that look like they meet up, but actually don't, thus keep a face from forming. Othertimes the bounding edges are too complex in their shape for SU to realize it should have a face, othertimes its layer and group issues, etc. The list goes on and on for what can keep a face from forming correctly.
A good work around it to draw a line that splits the floor in half. See if you can get 1/2 the floor to close proplerly. Then keep drawing more lines trying to get the next 1/2 to close. Eventually you will narrow it down to a small space that wont close. Zoom in really close and see if you can find lines that don't connect. Then connect them. Then delete all extra lines that you added. Or delete them along the way just to make sure its all working correctly as you go.
You can upload the model to the 3d warehouse and post the link here if you still need help.
Chris