One of the main reasons I see golfers struggle with bunker shots is down to the set-up they use before making their swing. Too often I see the shaft leaning forward and the hands too high at address. The sand iron is designed to work differently to other irons in the bag and this set-up doesn’t allow it to work effectively.
So lower your hands at address, make sure the ball is positioned forward in your stance, the weight is mainly on your left side and the butt of your club points towards your belly button. Make a normal, smooth swing, keeping your weight on the left side throughout, hitting the sand 2 - 3 cm or so behind the ball and completing your follow through as you finish forward towards the target. Accelerate through the shot.
Practice this regularly and you will develop feel for the distance the ball travels.
PGA Head Golf Professional