If your Driver and Woods are ok, I bet you have the case of the slides. You are getting away with it when you have the ball forward, and I bet your ball flight of the driver and woods climb upwards in an arc (where you are seeing some loss of distance).

The Irons are blocked because you have the ball in the middle and when you slide forward with your hips rather then just rotating them and staying balanced over the ball, you are moving everything forward and your center is now on your back foot = Block.