Still too little info to judge what's going wrong. Here's the general idea of vaping.
First, I set my device to watts, but use volts as a general guideline as to what setting it should be. Go to ohms law calculator.... plug in the ohms of the coil you're using, then plug in the watts you're using them at then calculate. You should be below 5 volts as a general starting point.
PG and VG.. PG has a consistency like water, and VG has a consistency like vegetable oil. Some tanks have a harder time with the thicker juices wicking fast enough. The Kanger tanks have no problem with high VG juices, but that depends more on the coils you're using. The older square type coils tend to do better with the higher VG juices, but the newer ones, I've heard nothing but problems from them. Once you burn a coil, it's pretty much toast, and you either have to coil/wick it, or trash it... In other words, once you dry hit it, it's ruined.
Now, as far as the juice, I already told you about consistency. There can be other factors in juices that can turn your coils black. If I use one coil almost exclusively, it will last about 2 weeks for me before it starts degrading. This is with a known good juice. Some juices may have some sugars in them, which caramelize on the coils. Others have particles from the flavorings. I've noticed that some of my first juices would gunk up my coils quite quickly. I eventually found some that extend the life of the coil considerably. I'd try a different Juice/Juice company to see if that helps a bit. There's really too many variables to pin it down to one thing. If in doubt, try pure VG... VG leaves practically no residue, so if in doubt, test with VG for a tank to see what happens.
Some other questions.. what juice manufacturer/flavor are you using? What's the color of the juice (darker juices tend to gunk up faster)? show us your coil... do you have the airflow open or closed? how long of a "drag" do you take? how do you prime the coils?
