I got a 7 month old that was not interested in bumpers, i finally got her to half heatedly retrieve a bumper in the back yard but she wouldn't in the field. But a couple episodes with clipped wing pigeons, then a shackled Mallard, then her first real field marks of any kind were fresh shot chuckars. Her third bird was 200 yards and she went like a bullet. Her next retrieves were fresh shot Mallards and a hen Mallard flyer at 150 yards and she pinned it.
She just turn 1 and we are now CC and she will retrieve bumpers with abandon. Some pups mature more slowly than others. Make sure you get the prey drive (chase) going before too much obedience.