It's probably your wicks. I got that ALOT when I was
vaping with 100% vg. Thick juices will definitely do that. I now use a 50/50 blend and have no issues at all. You have to make sure your tank sits a good 10 minutes before you start so the wicks suck up all that juicy yumminess. Every so often, puff on it a few times before hitting that power button. It should get those juices flowing nicely.
I'm using a nova with short wicks right now so I tilt it and swirl the juice around the wicks a few times here and there.
Also, hitting on it 12 times in quick succession is never good for wicks! At least it wasn't in my case... Got alot of dry hits that way (that's what that icky burny taste is called... a dry hit).