Excessive fines causes feeding issues, and excessively large coal can as well. A mix of rice and buck should be fine, but are you sure that is what you have?
One thing about burning buck, the amount of auger you have extending past the end of the pipe is critical. More than 2-3 auger flights will cause grinding, and/or broken shear pins.
Corroded feed pipes can cause the auger to turn hard, and so can a gap between the pipes.