I agree with what yzer just said. About a half hour after continuous vaping you would need to top off with more e-juice. A standard length cartomizer only has a 1 ml liquid capacity. If you vape it dry within a half hour, it will burn dry. And once a carto's filler is burnt, it's literally toast; you'll never get that carto working again.
Curious that you seemed to have the same problem with cartos in a cartotank. Did those cartos have pre-drilled holes, or did you punch holes in them? Regardless, it sounds like the carto wasn't getting enough (or any) juice from the tank.
What method were you using to fill the cartomizer and the tank? I've written a cartotank tutorial in my blog. It has a step-by-step method that I've been very successful with, and also includes a couple of videos on two methods to set up a cartotank.
http://www.e-cigarette-forum.com/forum/blogs/baditude/3710-5-cartomizer-tank-setup.html
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Having said all that, there is a slight learning curve involved with cartotanks that most of us endure. But following one of the tried and true methods generally results in a very satisfying tank experience.