Understanding sMaptool & Lowering map floor height

OK I have done numerous searches using different wording, however the most I have comprehended is to use sMaptool to open hmap for this. I am trying to lower the map for q20 from 343's release as mobs are falling through the floor. After loading up the hmap for this map in the tool all I get is a black picture and am at a loss of where to go from there.

Read elsewhere that it has to do with something in litmodels, but again I am unsure of this. Any steering would be great, don't need given a fish, just some pointers on how to use the fishing pole so I don't starve. Thanks in advance.

If mobs falls throw the ground, u need to check your server config. Min (0) and Max (800) height.