I've had issues with that too, and IMO there are two things that could cause that.
1. what's happening is you're getting too much power to the coil compared to how much
juice you're getting, remove a flavor wick or possibly rebuild or change the coil and it should be ok.
2. some juices seem to burn off weird at either too high or too low (cooks instead of vaporizes) a temp
when I use a protank, I use a chinese knockoff of the kick, so I can get regular, reliable power to it. Especially helpful because I rebuild most of my protanks with a 1.4-1.8 microcoil. I only run unregulated when I want to play around with sub-ohm and then, only in an RBA/
rda