Ahh yeah Rod's explanation might be the right one. Before you can use the clone tool, you have to set a point from wich you want the information to be first, and THEN you click somewhere to heal/clone it over there.
Select the clone or heal tool, now control click on the image where you want the tool to draw from, this is your source point. you should now see the outline of the tool where the source is. Now just click anywhere else on the image and you should be cloning or healing from the source.