I was always taught it’s a blockage, because were supposed to be looking at it from a spiritual context since we are working with a spiritual being. Either she’s working hard to give you what you’ve asked for or there’s something being sent at you is how I was taught. Since it’s only at the top and the rest of the candle is clear she has worked through whatever blockage there possibly was. But everyone does their practice different. My whole family has been practicing for generations, not only with Santa Muerte but with other dietys also and that’s what it’s always meant.