Assuming you have current GPU drivers installed, including OpenGL drivers, the code for the icing does what X-Plane tells it to do. If X-Plane says "add ice to the wings", the ice will start forming. In your case, the conditions, for whatever reason, simply weren't there. I know it seems like ice SHOULD have formed, but this comes down to the sim.
Post your log.txt file, just for curiosities sake so I can see what's happening.