Try another class. One puppy class won't be enough for most dogs or their owners. It's always easier to teach in class under controlled circumstances and then transfer to the outdoors.
At that age stopping made no never mind to Oban but changing direction worked wonders. I still have to do it sometimes, it sure wasn't like teaching SIT or DOWN which did take only about a week. It's an ongoing thing. He only pulls to get to something with a pulse? Hah!! Peemail has no pulse, favourite grass to eat has no pulse, wild fallen apples have no pulse and on and on, they all can incite pulling.