I kind of got into candle making in the last few weeks and decided it would be a great idea to make some for the family as christmas gifts but I kind of ran into a problem....
well, assuming you are using room temp glass (you didn't heat it up or cool it down, it could be that the was was too hot when you poured it, or the glass wasn't clean enough
When the wax cools it contracts and pulls away from the sides of the jar, what you can do is heat the jar slightly or make sure the wax doesn't cool too quickly