I know I'm a relative newb, but I think you aren't doing anything wrong except you payed too much for a KR808D-1
I think GreenSmoke is KR808D0-1 which is a respected model, good battery life with standard and xl batteries, good flavor (though some cartomizer types are hot - I like the Coolcarts at Vapor4Life which I think are Boge brand cartos. If your cartos don't burn your lips then they may be Boge type). If you need replacement parts, try Vapor4Life for nice colors and lots of prefilled cartos (I recommend their Coolcarts over the premiums). And if you just use black or white batteries, those can be gotten cheaper elsewhere as plain KR808D-1 batteries. Do not pay $50 for a battery from Greensmoke. Batteries should cost about $12-15, maybe $18 for a nice color at V4L.
I like KR808D-1 somewhat better than 510 too. I also think the vapor has more flavor than standard 510 carts and the fit and finish seems nicer, at least on Vapor4Life's KR808D-1 which they call "Vapor King". I had my boss try 3 PV's and he preferred my M401 and the KR808D-1 to the 510.
You might like the 510 better with Boge cartos rather than carts, but then it would probably be about the same as a KR808D-1 but with a shorter battery life. The main benefit of the 510 is its upgrade path and flexibility. You can use it with carts or cartos, and you can also use the 510's attys and carts (or cartos) with a really long-lasting battery setup people here seem to be in love with, the Ego. So 510 is a very flexible unit and suits a lot of people.
Bottom line - there is nothing wrong with liking with Greensmoke better. Some prefer KR808D-1 (same as Greensmoke but not jacked up prices). But you might also be using inferior ejuice with your 510 (like 100% PG juice which does not give much vapor and is a bit harsh) or not have it working optimally for some other reason.