I mostly use the 510 on the 801->510 adapter. When I use KR808's I use the 901-> 808/GreenCig adapter...it's a small tube that adds airflow. I get no vapor with my 808's directly on the 901 tip. I get more vapor from the 510 than the KR808. There were some 801's (BE112, I think) that won't work properly on the XH with the original 801 tips. Drew has these redone so they now work with all of the 801's properly. I don't use 801's much, but on the original 801 tip, I had to loosen the atty a little bit to get airflow. Try loosening it and see if you get vapor, but the best thing may be to get a new 801 tip from Drew.
Even if that's the problem, the 510 should still work on the 801 tip???
One other thing about the XH is to keep the contact point inside the tube and the end of the switch rod clean and wipe some Noalox or OxGard anti-oxidant in there....it makes the switch work flawlessly.