Not to pose as an authority, but Im pretty sure that the situation the OP describes---a tightened carto draw---is caused not by a hardware problem, but by the juice itself. Kentucky Premium is made with natural extract tobacco flavoring, and thats the cause.
Naturally-extracted tobacco flavoring is made (in the simplest terms) by a maceration process followed by filtering. Put some quantity (amount varies) of tobacco leaves (either fresh or dried/cured) into a container, such as a bucket with a lid. Cover the leaves with pure PG or VG. Thats the maceration. Let that stew steep (time varies, from a couple days to some weeks, assuming the maceration is kept cool/chilled to prevent spoiling). During the steeping, chemical flavor components of the tobacco leaves seep or leech into the liquid. When the steeping is adequate to color and flavor the liquid, pour off the liquid (discarding the leaves), then filter it through various sieves or paper filters to remove the tiny pieces of leaf solids that have ended up in the juice during disintegration. Those are called particulates. The resulting filtered liquid is natural extract tobacco flavoring.
Current filtering technology cannot completely remove all the particulates without also reducing the flavor, which was the whole point of the process---to extract the natural flavors, rather than trying to artificially synthesize them in a lab with chemicals. As a result, most natural tobacco extracts are dark-colored---because they contain some concentration of left-over particulates.
Many tobacco-blend retail vaping juices are now made using natural extract tobacco flavorings---Kentucky Premium is only one of hundreds, with more vendors using them as time goes by. Why? Because they taste better than synthetic flavorings. Also, natural extract tobacco flavorings can be purchased through various retail vendors for DIY juice-making. The two most well-known and ultra-concentrated natural concentrates are called Tobacco Absolute and Tobacco Extract. Most other natural tobacco extracts flavorings are made in-house by the method described above.
When loaded into filler-based cartos, the darker particulate solids in these juices will begin to collect near the bottom of the filler, gradually darkening it, and eventually clogging the filler batting sufficiently to tighten the draw. You can see this process occur by using a clear-tube carto. Over time and a number of refills or top-ups, the carto filler will darken gradually from the bottom of the carto up toward the middle. That part of the filler may appear almost black and sort of gunky.
Before the draw tightens, the carto can usually be cleaned by rinsing it out for a couple of minutes in a strong stream of very hot water. After such a cleaning, the renewed filler may not be as pristinely white as when new, but it will get to eggshell off-white or very light gray. After blowing out the cleaning water from the carto, it should be good for another bunch of refills. Once the draw tightens, however, the carto is dead and cant be saved. At that point, into the trash it goes.
Some juices made from natural tobacco extracts contain more particulates than others. For instance, VermillionRiver Classic Blend is much worse as a "carto-killer" than Kentucky Premium, which in turn tends to be worse than the various offshoot blends---Kentucky Applewood, Maplewood, Vanilla, etc., for the simple reason that VR's Kentucky offshoot blends contain a lower concentration of particulates than Premium Blend alone.
But so what? Cartos are cheap, and a small price to pay for the delicious flavor provided by tobacco juices made with natural extract flavorings.
