Problem with fix wall/region

Hello,

I seem to be having a problem using the fix wall/region command. My simulation entails building a trapdoor mechanism at the bottom of a region filled with grains. Thus far, I have created the mechanism by generating three "thin" regions that lie next to each other and act as the floor, and then applying the fix wall/region command to each of them to create the walls. My plan is to unfix the middle region at some stage, and the trapdoor mechanism would work.

However, whenever one of the granular particles touch any of the walls, I get an error saying, "Particle on or inside fix wall/region surface." I have since experimented with several particles and regions, and it always seem to trigger the error as soon as the particle hits the wall. How can I correct this? Is there a better way of creating a trapdoor mechanism?

Thanks in advance.