Clone stamp issue

Is it possible you are confusing the name of the clone stamp tool with a stamp-like action like that of a rubber stamp on paper.?

You could easily create a custom brush or a custom shape which would act as your "stamp" whenever you wanted to add a watermark.

Example of a brush:

Example of a custom shape:

To create a custom brush - Draw your watermark (or type) as you wish it to appear on a blank layer. Make it fairly large so as to accommodate various size images. Be sure to hide all other layers.

Then from the Edit menu choose Define Brush Preset. Give it a name.

Now the brush is available to you whenever you want.

To create a custom shape - Draw your watermark (or type) as you wish it to appear on a blank layer. Make it fairly large so as to accommodate various size images. Be sure to hide all other layers.

Command / Control click on the layer thumbnail. This will make a selection. In the Paths panel, click the icon in the middle of the icons at the bottom of the panel (Make path from selection)

Then from the Edit menu choose Define Custom Shape. Give it a name.

Now the shape is available to you whenever you want.

You indicated that you selected the watermark. It is not actually Selected and that may be the problem.

I assume the watermark is a separate file. Let's call the watermark the Source file and the file to which you want to apply it is the Target file. With the Source chosen, Alt-click on any point within the watermark. Then release the mouse and move to the Target file. Now click and drag the cursor to clone the Source image.

(One more thing: since you want the watermark to be an integral part of the image, it should be on the same layer as the image itself.)

Did you check the options bar? you should check Current layer and below aor all layers. You are probably trying to clone an empty layer...

That's the wrong tool for that.

See this thread for some ideas:

I have never used photo shop before, and I want to learn how to make a water mark for my photos