Because slim
batteries are not supposed to use LR cartos. 450mah+
batteries can usually use LR cartos. Get a 3.7v mod or the Riva 510 kits at Liberty-Flights are 3.7, or a clone Ego 3-stage at Gotvapes which does 3.2, 3.7, 4.2v and use 510 LR cartos. Or buy a 450mah+ variable voltage mod and crank up your KR808D-1 cartos with higher voltage.
Have you tired the Vapor4Life WOW cartos? Those are the hottest acceptable ones i know. I use a delrin drip tip as mouthpiece because they get too hot for my lips.