Although there are a few things that can cause this, one of the first thing to check is your grip. A "weak" grip prevents your arms from rotating correctly. If the handle of your club is too much in the palm of your hand, this will prevent the club from releasing.
Nestle the club down in your fingers, keep your grip pressure medium. A good check point is that the toe of the club should be pointed up about half way back and when you follow through, the toe will rotate over.
As you create the swinging motion, feel the tempo and rhythm. Hit some shots off of a tee with an 8 or 9 iron and try to become more aware of what the clubface is doing.