It does build up underneath the cylinder shaker, but I think there is such a small space between the sieves and the shaker pan at the back under the walkers that the stuff just slowly builds up, and then plugs up.
Sounds strange. Is this the space below the thistle finger on the back of the grain panIJ Do you have a thistle fingerIJ This would be at the top of the fan housing outletIJ Is your belt loose that runs the shaker pitmanIJ The late model 800 series had a plate under the thisle finger to extend the length of the grain pan and drop the material further on into the airstream.