OH...jumping back in here to add a comment about paper as well. As dini said, different paper take inks differently.
If you are using SU paper you will have the most luck with the Whisper White or Very Vanilla. They are "ultrasmooth". Once you get your stamp evenly coated with ink and apply the right pressure this paper will give you the best "smooth" looking solid areas of your stamps.
Using something like the Naturals White or Naturals Ivory by SU makes it harder to get a good "solid" .