Hi Don, the curve on the left is not flat with respect to the x/y plane like the other one is, that's why it behaves differently.

In order for a line to be used as a cutting object, it needs to be on a plane parallel to top, front, or right views of the construction plane. If it's just kind of angled vertically in space (even if only by a small amount that's somewhat hard to see as in your case here), it won't know which direction to extrude it in.

In the attached file, I've edited that line by going to the Front view and squishing it down using "Flat" snap (see http://moi3d.com/forum/index.php?webtag=MOI&msg=3378.4 for an example of flat snap), it should now work as a cutting object.

Hope this helps explain it!

- Michael