I newly started using shape keys to create an animation, which is just a cube wobbling on the spot, the first shape key that I applied was the top face of the cube rotating slightly, and to this point it went all good. Then I wanted to add another shape key, making the same face rotate back to place. When I created the Shape key, I went to Edit mode, but when I tried to rotate the face, it wouldn't move at all. I could see the dotted arrow as I usually would when rotating, while the face remained still.I tried using the hot keys (R -> Z), the Mesh tools -> transform, and 3D widget manipulator and all of them resulted in the same. I tried to do the same thing to the first Shape key and it worked, while the second did not, even though both seemed to have the exact same settings
You probably enabled the option which allows you to see the applied shapekey in Edit Mode.
Please set the shapekey value up to 1, and then edit the mesh or just disable this option.