It's possible that when the worm was welded, it now is not turning in a true circular motion and feeding the coal in a non-uniform manner. It's also possible that the flights are sharp due to age, which will also throw coal to one side or the other. Are the pipes tight together in the coupling; a factor that will cause worm binding if they are not tight against each other. I'm guessing that your worm is no longer turning in a concentric manner and that you have some grinding up of the coal. Do you pull the lever and empty the fines each day while the motor is running?