Misfit

I am new to candle making. After reading articles on the ins and outs of candle making I made my first candle. when the candle hardened I saw that it had pulled away from the sides. My second attempt at a candle lead to it slightly sinking in the middle near the wick. I had not checked the temperature of the wax upon pouring so I assumed that was the reason. The third Candle attempt I made sure the wax was at the correct temperature still the candle sank slightly in the middle. What is causing my candles to either sink or pull away from the sides?