Could be the little clip inside which catches on the other bit underneath (bit which connects to the motor, Forget the name)
Take the screw out the middle of the drivecone and remove it pulling it upwards, may have to wiggle it a bit, Look underneath, There should be a little bit of plastic about 5mm sticking out. If there isnt, buy a new drivecone or if you can be bothered, try putting a screw of somthing similar were it was