Terrain Question on Lower Height tool

I have made several missions using terrains that have large flat areas. On some of these, if I use the lower height tool, it works, no problem. On others, it will not lower the height any further than it is. I want to use lower height to make lake beds.

When it will not work, I notice that the tool tip says "average:0" which I guess is the source of the problem, I guess it doesn't like negative numbers. However, if I raise the entire terrain, I still can't lower the flat areas any further. I guess I do not understand the relationship between the xyz position of the terrain in the editor, and the actual altitude of the terrain. I particularly have this problem with imported heightfields. The terrains self-generated in Torque, although still flat, when I mouse over them with the terrain tool, the too tip will read something like "average: 24" . How can I import a heightfield with a flat area and make that flat area still be able to be lowered?