Based on my teaching experience, most golfers that struggle with the ‘Chicken Wing’ or not achieving full extension is because they are using their arm strength to move the club and not allowing the natural swing motion to develop their full arc. By lifting the club and arms to control the club, the player will invariably develop a Chicken Wing that is difficult to make for a consistent swing.
As for lacking full extension, this is another bi-product of controlling the arms and club, rather than allowing them to swing. My best advice is to take a golf club (with or without added weight on the clubhead) or use an Orange Whip to slowly, methodically swing the club or Orange Whip back and forth until you develop a swing arc. Once comfortable, you can gradually pick up the pace to develop your swing with full extension and no Chicken Wing! Good Luck golfers.