WHAT IS A SINK HOLE?
Have you ever lit a candle and it melts straight down and can never melt the whole surface of the candle? This is probably because there are air bubbles in the wax.
Sink holes are tricky because you usually can't see them. They are most commonly under the surface, near the wick. Any movement of the candle while it's drying can cause air bubbles to appear. It can also happen if the room in which the candles are setting up is too cold. Cold air causes wax to shrink.
In order to avoid this problem completely we use a heat gun to melt the surface and reveal any air bubbles under the surface. If there is one we melt the surface until the hot wax can fill in all the pockets of air.