In order to use the clone stamp tool you need to first target an area to use as the source for the clone stamp. After selecting the tool, move your cursor over the portion of the photo you want to use as the "source" and holding the ALT key down (on the PC) and click. Then move your cursor to the area you want to clone over, and click.
If you are already doing the above steps then try to reset the tool.
In PSE 11 Select the clone tool and then on the right hand bottom expand the drop down, refer the below screen shot
And then select the option reset tool