I've had that happen when trying to make layered votives. As Top said, you've probably let the first layer set up until absolutley cold. Just let the first layer set up until the top of it is solid and the sides are sticking to the mold, but the layer hasn't hardened right through. Then pour the next layer hotter. I also hit the top of the first layer with my heat gun for a few seconds (not enought to heat the layer) then pour the next layer. You might want to practise this technique on a couple of votives first, so you don't have to keep melting you candle down.