Candle Tunneling is when a candle burns down the centre, leaving a rim of hard wax around the outside. It will result in your candle not burning for as long as it should and you may even find you lose your wick eventually. It's frustrating and can happen to all candles regardless of the quality and cost. The good news is that tunneling isn't inevitable and it's completely preventable. It's all about treating your candle the right way from day one.
The key to avoiding tunneling is to leave your candle lit for 2 - 4 hours or until there is a full melt pool the first time you light it. You need to ensure that the wax melts right across the top of the candle on the first burn. This creates a candle memory that ensures on subsequent lightings, the candle will continue to burn right across the whole surface right down to the base of the glass.
Throughout the life of the candle, it's best to avoid frequent short burns, instead ensuring that you have occasional long burns similar to the first one, to keep the wax open.