I am using PSC6 and the clone tool stamp will not paint from the sample area. After holding down the alt key to obtain sample it will not paint. I can hold the sample over the area to paint & see the cloned area visually, but it will not paint when I click. However if I click many multiple times I get a faded clone.
Chuck the opacity and flow of the clone stamp tool in the main menu bar. It sounds like it's not set to 100%