‘Tunnelling’ is where the candle burns down the centre leaving a large amount of unused wax on the sides. If this has happened with your MIXMY candle or refill, it is very likely due to not being burned for long enough, particularly on the very first burn.
Your candle should burn long enough for the surface wax to melt completely to the edges – generally between 2 and 4 hours.
This is especially important on the very first burn, because if you don’t, you’re almost guaranteed to get tunnelling. This is because wax has what is called a ‘memory’ – so even when wax appears to have solidified, it will continue to harden over time. Wax that has melted and cooled again yesterday, will always be softer than wax that has been left for longer.
So if you light your new candle and blow it out before the wax has melted to the edges, the second time you light the candle, only the softer wax in the centre will continue to melt as it requires less thermal energy than the much harder wax around the edge – resulting in tunnelling.