Ask your broker about it. When you experience such a large slippage, it is usually a result of a large, quick movement in the market. The chart wouldn't show a gap, but the candle at the time you executed the position would be at least as large as the amount of slippage, minus the spread. If this is not the case when looking at the chart, you should contact your broker to find out what happened

All you have to do is to send a letter to your broker to forward your concern to the designated department to consider your claim.