They are currently out of stock.
I don't use LR because on the Provari it makes more sense to me to use the higher resistance coils. But it may be that if you like a hot vape you need the LR cartos - I can still get it pretty warm, though, by turning up to 4.8 or so!
I run 30/70 pg/vg juices through Smok dual punched single coil 3ohm cartos on my MVP2. I usually run between 7 and 8W They last me anywhere from a week to 3 weeks. Depends on juice, usage, power applied. As far as the OP question. Could be a number of things. If I get burnt taste repeatedly after turning down the power, or tight draw, or little to no vapor then I change out. I am not too tight on cash at the moment, fingers crossed, so I like to keep 10 to 20 on hand so if i need to change out I can. The cost of a carto and its lifetime dramatically out weigh the price on analogs. The more you fiddle, the more you will be in tune with them. Too bad you can't rebuild them... ...yet.
Just put the new one in from the top and push it through while flipping it over. Top becomes the bottom, prime and you are back in business.
ETA*** Always make sure you back up the cap that is going to try and push out when sliding a cartomizer through the tank for a swap or when filling rather than holding the tube