Glass texture problem?

The Siphon

L6: Sharp Member
Mar 2, 2015
278
190
I was mapping for the 2016 spawnroom contest. I created a window as seen in the video, but when I fired an arrow at the right window, the left window turned black/dark. The glass material is glass/glasswindow001a on the other side and icarus/glasschrome003 on the other side.

I checked, there were no problems with the map or any leaks. Is this a common problem? Will it continue happening if I add lights, finish detailing... ?

If you can help me, thank you.