Terrain's heght not increasing

I have created a terrain from a heightmap with the help of terrain tools. Initially I set the terrain height to 300 terrain units. Now if I try to raise my terrain further using the raise terrain tool I am unable to raise the terrain further .If I increase the height of the terrain the height of the entire terrain increases. I want the physical height of the terrain (which I created from the height-map) to remain 300 units, but I should be able to raise the terrain further using the terrain scaling tools.

Your help will mean a lot.

sorry, its not possible.
But you can increase the height if the terrain and also lower the contrast of your height map could work.
try increasing your height to 600 and also manipulate your heightmap to lower its contrast to half.