craftgrrl | Candle-making questions.

A friend and I got together for a craft day yesterday. After spending three hours in a mall to find all of our supplies we finally had everything we needed. Who knew finding everything would be so hard?
Anyhow, the soap and lip balm went great. However, our (first ever) attempt at making candles did not. The wax around the wick concaved and created a rather large valley in the center.

My question is for all of those who have experience making candles, why did this happen and what can we do to prevent it when we make candles again?

Thanks!