[photoshop-beginners] Clone tool

This morning my clone tool started acting up. CS2 I can clone white objects say sky over a dark wall. I can not clone a colored object over another color object. I can kind of clone a dark object over the white, looks faded ! Just does nothing on a wall to cover a spot or something,. Worked fine earlier.. Checked and have only the back ground layer.. Been cloning for days with no problem.. Have the correct tool, tried changing the brush size to no avail...
Rebooted the computer, turned off Photoshop, brought up another photo.. Its broke. All that's left is reinstall Photoshop but thought I would ask here first !
Help ! Thanks , Steve

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