This happened to me many times when I began to make peppermint bark and I think it was because I left the first layer of dark chocolate in the fridge for too long. When I poured the white chocolate on top, the temperature change would shock the white chocolate and make it diffucult to adhere to the cold hard dark chocolate. Some recipes call for no fridge time at all but I find that the chocolate can take a while to set. I place my dark chocolate in the fridge for about two minutes after its been added to the pan. This gives it a quick cool down but doesn't make it cold enough to harden it completely. The dark chocolate layer can still be a little wet when you pour the white chocolate on top, just don't press down too much or you may mix them together. Once the white layer is on top then you can place the pan into the fridge to set. Bring the peppermint bark to room temperature before breaking into pieces.