Tunneling is when the wax only melts in the middle, creating a “tunnel” while the outer wax stays solid.

Why is it happening? 

Tunneling can occur if the candle isn't burned long enough on the first use, allowing the wax to melt to the edges (typically a 3-5 hour burn). This initial burn, known as the 'Memory Burn,' is important for consistency later on. Other factors can also create tunneling such as improper wick trimming, airflow or drafts. If there’s a thin ring of solid wax around the edge, not a problem, it usually will work itself out on future burns.