Sometimes Maya can't use certain curves. I run into this with certain fonts and whether or not they are bold or capitalized. Ususally, and by no means am I going to try and be technical, I will adjust some of the curves control points so that they are in the proper position. Sometimes they will over lap. You may be able to fix it by rebuilding the curve with more or less control points, or even change the curve direction. You also have to make sure they are planar. and on the same plane.

I hope that helps a bit.