It might be a problem with the actual brush type.
Select the Brush Selection tool (somewhere near the top of the screen) and make sure that you are using a brush with a hard edge (unless you want it feathered)
The cloning tool requires that you have selected a distance to copy from. Press and hold the ALT button while you have the cloning tool selected. Now click on the area of the photo you want to copy, and then click and hold the area you want it copied to.
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