Hi Brian, the problem here is your curve on the front side was not completely closed, it was actually had a small opening where 2 segments were not completely aligned end-to-end:








To tune this up, select the curve and use Edit/Separate to break it into segments, then turn on control points for one of those segments in the problem area, and drag its end so that it snaps directly on to the other segment's end making a fully closed connection. Then select the segments and use Edit/Join to glue them back together.

Then with a fully closed outline your shape should get capped.

- Michael