I personally like chalk inks with solid stamps. The ink is thinker and tends to coat the stamp well. Otherwise, I would have suggested the steps you have already taken (cleaning, sanding). You could also try getting a better stamped image by using some extra padding, like a magazine or piece of thin foam, under your paper when stamping. You could also ink up the stamp, lay the stamp flat on the table and place your paper on top. Gently rub over the image with your hand, or use a brayer. This will help to get the paper in full contact with the stamp. I do this with large or background stamps that give me problems stamping a full image. If some of these suggestions don't help, is it possible that the stamp itself is not even, therefore, being defective? Good luck!