Thanks for the suggestions. I was able to solve the problem, and in so doing may have found out something about sketchup that may help others.
I was never able to close the face "normally", so I tried Wo3dan's suggestion of a diagonal line. That didn't work either (the face didn't fill at all or only partially filled.) However, on closer examination, some of the lines between the points, while appearing to be continuous, were actually combinations of multiple segments. So I erased the segments, replaced with a continuous line and it filled diagonally, which is ok for my purposes. I still don't understand why it won't fill normally with two Z1 and two Z2 points, but at least this worked.
So to make sure a face closes diagonally, make sure the lines are single and continuous and not combinations of segments.

Title sums it up, I have three faces on a three planes of a stadium, they are downward sloping at 14 degrees. The z coordinates of the two upper points are the same, as are the z coordinates of the two lower points. Yet it won't close; all of the other planes of the stadium do close. I ran MakeFaces and it doesn't close it either. Any ideas would be appreciated.