Tunnelling
When a candle doesn’t burn evenly and leaves a lot of wax around the edges of the container, it’s known as tunnelling.
There are two causes of tunnelling. Either it’s user error or maker’s error.
Did you burn your candle on first burn long enough to have a full melt pool and so reaching the edges all around? If you have very small amounts of wax left at the sides they will melt away as your candle burns hotter lower down. The second reason for tunnelling is that the candle has been wicked incorrectly.