I've found that a lot of times, bad juice or certain flavors really mess them up... Heavy dark flavors can be the worst, but not always... I've not found a grape that won't clog up the works..
My favorites have become floral extracts. I find they don't burn very easily, and they produce a lot more vapor...
Those are reasons I got into mixing my own juices... I know what I'm getting for the most part, and I can afford to find what works and tastes the best.. It takes Vaping to a whole better level....
I don't end up with as many toasted and trashed attys because of bad juice..
OH, an I can control the fact I can get it down to 0 PG... I had a lot of dehydration and skin problems using PG not to mention irritated throat... nothing but VG now.. it also produces a much better vapor. Most places use flavoring that is PG based so you still get some PG even if you order VG.. most are 80%
I've also found that VG doesn't foul attys as much, nor does it leak and gurgle as bad..