The main thing that will cause this to happen is switching from dumping out a wax melt and adding an oil that is cold to the hot aroma top.
NEVER ADD COOL OR COLD OILS TO YOUR AROMA TOP WHEN IT IS HOT
We recommend to always add wax to your aroma top and then add some oils to the melted wax. The wax will absorb the coolness of the oils keeping the aroma top safe.