I have notice that in the latest version of Photoshop the clone stamp tool and the healing brush tool won ́t work properly if you have done previously a curve adjustment layer. You have to deactive it first. I guess it's a bug.
Which specific version of Photoshop are you using?
If you are targeting an adjustment layer, then the only thing you could be healing or cloning is the layer mask for that adjustment layer. So it might help if you describe what you are doing with a bit more detail.
Opened again the file to check if I was doing it right, Seems that the origin of the problem was that I had "All layers" activated in the clone stamp and healing brush tools while doing retouching on the background layer. I have attached a snapshot of the layers window. Thanks!