We used to have a lot of issues with this and realized that it was usually due to dried pigment around the inside edges of the container. We now wash the tips before each use and it has helped immensely. Using a drill press for mixing should help a bit as well if you aren't already.
White and black pigments have always been the most troublesome in my experience, and I stay away from white pigs altogether these days. Matsui's 301M is a much better solution for standard WB inks and we use white discharge ink in place of white pigs for discharge mixes. As for black, we'll go as far as to transfer pigment containers before mixing to ensure none of the dried pigment gets into the ink.