One of the most common (and forgotten) questions in making gel candles is "Can I use any candle fragrance in my candles?" The answer is no. Gel-safe fragrances must be completely oil-soluble (non-polar) to be used with gel candles. "Polar" fragrances have some component that is not completely oil soluble. They can cause the gel to break down and/or form small pockets within the candle that can be quite flammable (lower flash point) when the flame hits them. This can cause dangerous flare ups in the candle.
The easiest way to ensure that your fragrance oils are appropriate for gel candle wax is to buy them from a reliable supplier who has certified or listed them as "gel safe." But if you don't have that assurance, or (like me) have fragrance oils from past soap and candle projects, there is an easy way to test for polarity.