Well I mean, if an ND filter does give your photos more contrast, then it's not a very good ND filter.
It's right in the name... "Neutral density filter." That means by definition the effect of the filter on the image should be neutral. It shouldn't be accentuating or diminishing any specific part of the image. All NDs are meant to do is lower the total amount of light coming into the camera, not affect the makeup of the image.
You might try a CPL for what you want to do, but don't waste time with NDs if this is your goal for using them. They are specifically designed not to do what you are looking for.