One of the problems I see with the everyday player is the start of their backswing. What I see is that the right elbow gets in behind them. We call it the 'Lawnmower Start' almost as if you are starting a lawnmower.
Stop Drifting Your Arms
The arm drifts in behind your body very early. From the side view you see that the elbow gets trapped behind you and that pulls your left arm across and makes you have a very flat swing with your arms. Now you still can get the club up but your arms are trapped a little bit behind you.
Drill: Take Control of Your Backswing
Here is a drill to stop that. You can do this on the range or at home. Simply take your left hand, for a right-handed player, and put it under your left elbow so that it is locked in.
Then when you go up to the top of the backswing just practice your right arm swing going up to the top. There is no way that you can get it too far to the inside with your left hand tucked under your right elbow.
Correct Elbow Position
As you practice that if you feel your hand slip off your elbow then you know you are reverting back to that inside takeaway. At first you might feel a little pressure on the elbow, but pretty soon that will be easy for you. When you have practiced this enough then go take a club and go hit some golf balls. Hope that helps you.