It could possibly be as simple as you have an air pocket just under the top of your candle. Some waxes, even though they are one pour, you will still get air pockets. I know I still do sometimes, but sometimes it's easy to tell because you get cracks in your candles. What you can do is take a skewer or something of that sort and poke a couple of holes around your wick when it is setting up to release any trapped air, then either do a repour to fill any holes after it's hardened or hit it with a heat gun. I haven't worked with the type of wax you are using, so I don't know if it is prone to air pockets, I'm sorry... maybe someone else might know. But at least it's something to think about.