How many times do you hear your fellow players saying that they have such a nice practice swing but as soon as there is a little ball their swing is not the same anymore?
Most people have good practice swings, but they ruin it when they want to hit the ball towards the target. The reason for this is when you make a practice swing, you are focused on the swing, when you hit the ball, your focus is on hitting the ball and the outcome.
I encourage you to go to the driving range and make five practice swings, then place a ball and try not to hit at the ball, instead make a golf swing and let the clubhead hit the ball, only because the ball was in the way of the traveling clubhead.
The last two golf lessons I gave the players indicated that they are good iron players, but really struggle with their drivers. I explained this concept to them and instantly they were hitting the ball better.
When you hit at the ball your swing changes, your path and angle of attack is not the same as when you make a practice swing!
Head PGA Golf Professional