Make sure there are no accidental selections in effect. I struggled and struggled with clone tool. It now works. I think I may have had an area selected without knowing it. Having unintended selections may have been what caused so may tools to appear to not work for me. I have the unfortunate GIMP habit of canceling a selection by making a small, new one. You can be sure you have no selections in effect by using the Deselect function from the Edit menu.