Global Illlumination issue with modular level

I created a modular test level and I decided to test out precomputed GI. I have a lot of walls stacked against each other, however I see that the GI is not computing correctly with some of the double wall areas.

Also I happen to see GI lighting gaps between some of the areas where the walls are right next to each other.

Is there anyway to fix this, or will I have to model new walls?