If you were vaping for 6 hours then you could have gone through 4ml of juice with a vivi nova. I use the 3.5ml vivi novas and would easily vape that much if vaping for 6 hours.
You may have some small seepage going on - that can be from it not being threaded quite right when screwed on, as some-one else mentioned.
There are different types of clearos - the vivi novas are a clearo tank. I've been vaping over 9 months now, and while I like cartomizers I prefer the vivi novas.