A lot of kayfun clones don't have the vacuum system properly sealed. I suffered through that for months trying to figure out what I was doing. The usual source is through the 510 pin, particularly clones with adjustable pins.
Put your finger over the airhole and suck in through the mouthpiece. If your getting even the slightest bit of air moving, you have a vacuum leak. It tends to be so slight, you may not notice it at first because it won't be enough to cause a straight up leak while vaping. It will be enough however to cause slight leaking while filling and poor wicking which leads to dry hits.
Yes clones are another variable among many to simply say you suck at building is a bit nearsighted. My guess is most comments like that are vaping a light in the loafers juice like raibow candy cane or fruity pepples, etc