There are slight variations in both the individual cartridges and individual atomizers, so some will fit tighter than others even of the same type. This is why sometimes an e-cig.com or Crown 7 cartridge will fit an Njoy/Myecig/Smartfix, and sometimes not. But even when if "fits" it's not a good fit, and the device won't work at its best. Same for a Janty/E-Puffer cartridge.
So far as I can tell, Njoy, Myecig, Smartfix, and Pillbox are all of the same model, and should be interchangeable.
E-cig.com/Crown 7 is very similar and will fit poorly, but will work in an Njoy etc, just not optimally, and vice versa.
Janty/E-puffer is a bit more different, and while their carts will work in Njoy (again, not optimally), Njoy etc carts will not fit in a Janty/E-puffer.
I've been meaning to do a video review of the Janty KISSbox, buit I've been migrainy lately. Tomorrow morning after work I should be able to get to it, or maybe Wednesday if I'm too tired. But I'll also do a comparison video showing the carts.