Another reason is your wick size. If the wick is too small, the candle won’t be able to generate enough heat to melt evenly on the surface, but if it’s too long then it can cause it to burn ...

With a lighted candle, melted wax travels up the wick. When the wax reaches the hot flame along the wick, it vaporizes. The heat from the flame is hot enough to cause the vaporized wax to oxidize ...