There is one reason that golfers hit the middle of the ball, typically called “blading”, and this is because the low point of their swing is in the wrong spot. This can be caused by either bending the elbows through impact or not getting enough trail shoulder tilt, or both.
A great drill to help solve this is to place a business card on the target side of the ball. Focus on swinging through the ball and getting the business card to fly up into the air. This will get your low point in the correct spot for solid contact.