Wow, I went over the manual and there's nothing in there about hi-hat setup. I suppose you could try a few things.. first try another voice/kit and see if the hi-hat works better with one over another.. I found some of the voicings on my DM10 just seemed a lot poorer in how they played and sounded than others.
2nd, what about sensitivity, threshold etc? Perhaps some fine tuning or trying higher or lower settings will help. 3rd, is there a chance you've got the pedal and cymbal cables reversed (don't laugh, I've done it, and it produces some interesting results).
Lastly, it could be the pedal itself.. there have been many discussions on the realhat pedal's functionality.. some people changed the spring tension or how much play there was in the board as it is pressed down and so on. You may find something here if you do a search on realhat pedal or pedal sensitivity or something like that. In any case, there's usually a tiny screw at the end of the pedal (the control side, not the hinge) that you can adjust for how much play there is when pressing down.. maybe you need to increase or decrease it so that it senses the pedal is more fully closed when you step down? Just a thought...
Good luck, hope you figure it out, let us know if you do