There's your problem. Ease the back stick as you approach the top of the loop (nose about 30 deg above the horizon) and fly the plane across the top of the loop, applying more back stick as you start down the backside of the loop (nose about 30 deg below the horizon..I usually ease some power, too, to keep from getting excess airspeed/g down the backside). This will make a nice round loop, not one 'pinched' at the top.

BMatt nailed it right on the head.