Photoshop Clone tool does not have any effect

Hi,

I am new to PSE. To my disappointment, the clone tool would not paint sometimes. I follow the drill by picking a sample (Alt+Click), setting brush size etc but the damn thing just would not have any effect on the photo.
Sometimes if I close the picture and reopen, it paints.

Please advice before I am driven crazy by the freaking tool.
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'A photograph is memory in the raw'