This sometimes happens, even if rarely, as it can happen with all types of candles. For example in our case, if the coconut shell is too big or irregular. In jargon it is called "little well". To fix it, the trick is very simple: just keep it on for a couple of hours with the shell wrapped in aluminum foil (leaving the flame free). The heat will thus remain trapped inside the shell and the wax will melt evenly again.