Some golfers stroke their putts so that the ball gets into the air and bounces a little before it settles down and starts to roll. The bouncing can send the ball well off line.
If this is your problem, try placing the ball a little more forward in your stance. Bring the putter back low to the ground and strike the ball on the upswing. The ball should turn over and begin to roll immediately. Practice this technique to see if it is right for you.