Your questions, answered.
Candle wax shrinks as it cools from liquid to solid. After the initial pouring, the liquid wax will be in contact with the glass. As the wax cools, it shrinks, and the solid wax will break away from the glass surface as its volume decreases.
Clean Jars/Pre-heat Jars
Making sure the inside of the glass is clean and free from dust and grease can help promote adhesion.
Pre-heating candle glasses will improve glass adhesion, but this is normally only temporary, and the wax will usually pull away from the glass over a period of hours, days or weeks.
Pouring wax at the correct temperature can improve glass adhesion in some cases. However, wax shrinkage is an inherent property of the wax and cannot really be avoided completely, so most waxes will pull away from the candle glass to some extent.
The room temperature should be greater than 20° when pouring candles. If the ambient temperature is too low, the wax will cool and shrink more quickly, and pull away from the glass more readily.