There are a few different things that you could be experiencing. Most of the time I see this issue is when the screws in your posts are not tightened all the way down, so double check that all of your posts are nice and tight. Also, one thing I have noticed with any of the Isticks we sell in my shop is that for some reason every one of the tanks we sell that are supposed to be a .2ohm coil, (the Arctic by Horizon Tech for example) always say that they are a .3 ohm coil. I am not sure exactly why they all do that but they do with the 50 watt isticks. Another thing that may be happening is that when you wick it up and put juice on the coil it can sometimes go up a tiny bit.
I would recommend you build a little bit higher ohm coil for istick, I believe the lowest it goes is .2 but they always say .3 on them.