On one of my projects, I tried to render my wireframes using contour rendering. I followed the instructions from Ayan Ray’s post “The best way to render wireframes in Maya“, but didn’t have any wires showing up. After troubleshooting, I found out that no wires would render when using Unified Sampling. Switching to regular Adapative sampling rendered the wires on some objects, but the rest just had the outline wire. I quadruple-checked that I had both silhouette (coverage) and all poly faces checked in the contour rendering, and still no luck. After staring at my scene for a bit, I realized the ones just rendering silhouette outlines had Subdivision Approximation applied to them. After taking it off, the wires rendered fine. In short, it seems that both Unified Sampling and Subdivision Approximation don’t mix with Contour rendering. Case you need to render your wires, switch to regular adapative and turn off your subdivision approximations.