1. Melt pool
The first time you light any candle, you should let the flame burn long enough to create a large "melt pool" that fills the top of the candle. The melt pool is essentially the wax on the top of the candle that is completely melted. This should take 3 - 4 hours to create if the candle is properly made. If it happens too quickly, your candle's wick may be too big. If the candle never reaches a full melt pool, the wick may be too small. If you blow your candle out too soon (or the wick is too small) without allowing the melt pool to reach the sides of the candle vessel, you could cause a tunneling effect in the candle. Essentially, the candle will continue to only melt in this space, sort of like it has melt memory. See the tunneling tip below if your candle has this issue.
This candle has a full, clean melt pool. Notice there is no soot or residue on the sides of the candle and the flame is not too high.
2. Flame is too small
If the flame on your candle is too small, it is probably not creating a large enough melt pool. This could be caused by two different things: 1. the wick that was used to create this candle was too small, or 2. the wick is trimmed too short. You should trim the wick to only about 1/4 inch tall, no less.
3. Flame is too big
If the flame on your candle is too big, it is probably burning too hot. This can be a safety issue. It is likely caused by using a wick that is too large for the vessel of the candle. If the flame is too big, you may also notice soot or black dust in your candle. If you have a candle with this issue, either discontinue use or be sure to trim the wick extra short.
4. "Mushrooming" wick
Mushrooming wicks are very common, and not necessarily a "bad" thing. Some wicks are designed to mushroom or curl in order to "self trim" (many people don't actually trim their wicks!). If the wick on your candle is mushrooming, trim the wick down low enough so that the mushroomed pieces are cut off. Mushrooming can be mostly prevented if you keep the wicks trimmed properly on your candle. Some other things you might notice if your wick is mushrooming would be soot residue in your candle or too high of a flame.
This candle has minor wick mushrooming, which can be fixed by trimming the wicks.
5. Tunneling
Tunneling is probably the most dreaded issue with candles. A candle will tunnel for multiple reasons, but the main reason is improper wicking of the candle. Candles that have not been thoroughly tested while being made may be created with the wrong size wick. In this case, the wick would be too small to create a flame hot enough to melt the wax into a full melt pool. Other things that can cause tunneling are air pockets in the wax that weren't seen by the candle maker, not allowing the candle to reach a full melt pool on its first burn, or other improper manufacturing practices. If your candle is tunneling and you'd like to resurrect it, one way would be to take tin foil and wrap it around the top of the candle, keeping the top open. Light the candle and allow the candle to reach a full melt pool before blowing it out. Unfortunately, though, if the wick is too small for the vessel, there is little you can do to prevent or fix tunneling.
This image shows an example of tunneling.
