Terrain "Smooth Height" tool not working

Hello everyone. Sorry if this question was already answered before but I couldn't find anything that fixes my problem. I'm having troubles with the smoothing tool in the terrain editor, it's simply not working. When I select it and apply it on the terrain, it doesn't do anything. The Raise/Lower, Paint Height and Paint Texture tools are working perfectly, it seems that the Smooth tool is the only one that doesn't work. I tried creating a different scene with a different terrain and the result is the same.

Does anyone know how to fix this? Thanks in advance.