The way a flame moves into coordination of what you are doing is pretty significant. If the flame is dancing and seems to be wavering, i.e. as in not sure wether it wants to stay still or go out, means that your concentration is so-so. So , for example, you said that you were doing a protection spell. Since the candle flame was moving, the spell has a 50-50 chance of working , via the flame, that is aside from wether you did it right or it worked.
If a flame is strong and still - Your personal power & concentrations is good
If a flame is wavering- You need to stop and figure out what is bothering you, if you are fully concentrating or if it's right for you....
If a flame is low and wavering- Then you need to stop and try again another time, either when you are ready personally (learned more ), or you aren't concentrating enough
Hope that helped