Hi Andrew,
I believe this is an issue with the version of the program. When walls are smaller that 50″, no light would be produced on them. If you were to extend the wall passed 50″, light would appear. I tested walls as large as 15″ and they produce light in the new version. As a solution, I would suggest to update to the latest version of the program to have this corrected.
Click HERE to download the latest version 13.4.
Regards,
Kris