I had the opposite happen. When I switched to pellets, I mixed the two to ease the transition. Mine picked out the pellets and left the crumbles behind. I had to not add more pellets until they cleaned their plates so they would eat the crumbles. I did not want old food staying in their feeder and getting old and losing its nutritional value. We all have different experiences.
You can try mixing something they like with the pellets and they will probably learn to eat them. It's hard to imagine chickens not chowing down on anything that is food once they realize it is food.
Most everybody lets the rooster eat whatever the hens are eating. You can't stop it anyway. I recommend not adding extra oyster shell to the food and forcing any of the chickens, expecially the rooster, to eat more than they need. If you offer extra calcium, offer oyster shell on the side so they can eat more only if they want it. Sometimes you'll see the rooster eating extra too.