Ceromancy is one of the world?s most ancient forms of divination. Starring at a flame and viewing wax drippings is a great method of determining if your magic is working effectively. Additionally the way a candle unfolds or deconstructs as it melts can be very significant.
For your case I think wax residue means
When a candle leaves wax residue: If there is around 1/2? (1.27 cm) or more in the bottom, the spell needs to be repeated. Wax on the sides of the glass indicates where there are personal hindrances to the progress of the spell
You can check out the link to relate more since You know best about the behavior of the candle during and after the spell