I’ve discovered that Maya 2015 can have an issue with batch rendering contours for things like wireframes. When I went out to find a fix I couldn’t find much at the time. So instead I poked around until I figured out a quick solution.
If this solution doesn’t work for you I suggest exporting your file out as an .obj and reimporting it back into a fresh maya scene.
Below you will find a video tutorial with the fix, followed by a quick image of how to setup your shader group for those a bit more knowledgeable.