This is a common phenomenon referred as “tunneling.” This happens when a candle burns down the middle of the jar, which leaves lots of wax stuck to the side of the jar container. It happens when you keep your candle lit long enough before putting it out and can be avoided by keeping the candle burning at least until the wax pool reaches the edge of the container. It is ideal for the candle to be burned for at least 2 hours on subsequent burns, but never longer than 3 hours. Following these steps should prevent your candle from tunneling.