If you make a door or window with glass in Sims 3, or you make a mirror and you find that the glass part of your mesh is missing, this often happens when you have made your own glass or mirror replacement. This seems to be a problem with only a selection of windows and mirrors. Unfortunately, mapping the glass to match the EA original mapping doesn’t necessarily correct the problem.
Another symptom of the problem can be that glass CAN be seen, but for some reason, it is white or opaque rather than see-through.
If your glass (or mirror) is correctly assigned to a joint, then the only other known cure for this phenomenon is to use the glass or mirror found in the original EA object and reshape that. This will correct the problem for you.