Slojas's idea is a pretty good one and would help out. But there is another way. I don't know if you are using automatic or manual batteries. However, if you are using manual ones, try pushing the button first and then take several lighter, longer pulls. That will reduce the amount of air and give you more vapor. I noticed the same thing when I would use the 510 batteries. I now use Riva's, Kgo's, and the Ego VV 900mah batteries. The nice thing with these is that they have cones that cover the base of the cartos and allow slightly less air in. Without the cones on, it is exactly like you describe.
As far as the "D" in the 510-D, It only designates the carto as disposible, although that is pretty stupid IMHO. Al cartos are disposible. Even though it is designated as a disposible, it can still be used after refilling several times. I never saw any difference in the Boge 510's or the 510-D.