I have experienced this problem frequently. Often the clone brush does nothing. Occasionally it will paint at (seemingly) reduced density, even with all the settings at 100%; on one such occasion it suddenly started working OK.
I reduced the brightness of the attached jpg file, and then tried to use the clone brush to eliminate the scratches. It did nothing.
I reloaded the file and tried to remove the scratches, and it worked fine. I then reduced the brightness of the picture, and after that the clone brush ceased to work. This second file I saved as an afphoto file (also attached). When reloaded, the clone brush still didn't work, suggesting that something in the file is preventing the clone brush from working (i.e. as opposed to a fault which only manifests itself when the program is actually running).
A quick test suggests that use of any of the adjustments (levels, white balance, etc) prevents the clone brush working, and this would explain why I've had problems in the past. Painting a line on the picture (for example), or using other tools, does not appear to cause a problem.
Negative0000.afphoto
