Stamp tool used to work reliably in previous versions of DT. In 3.0.s it is very flaky. See the attached screen shot. I drag the circle to a clean spot and released the mouse button, still spot has not gone. I clicked on some other module(other than stamp module) still no use.
I just figured out that opacity is problem. Please see the attached pic. Actual opacity and what is shown in slider does not match. Mostly it is my mistake, I might have pressed CTRL key and that reduced to actual opacity. I came to know about actual opacity being so low when I liked at text.
There was a discussion about this issue some weeks ago. Look at the text overlay at the top of the image. Set feather size and opacity properly. You need an opacity of 100 %, as pphoto already said.
If opacity is at 100 percent and the issue remains, you can check the parameters manually. The parameters for the tools are stored in ~/.config/darktable/darktablerc (on linux). Quit darktable and edit the settings with a text editor (backup first !!!). Search for the discussion I mentioned ( darktable 3.0 masking inconsistencies & bugs ), there you will find further information.