Wet dirt on edible bean roots can give the same problems. What a nightmare for the harvester. To help rid machine of the dry dirt many have fabricated a screened door to install in place of existing rear feed door. Sloted screen is certainly the best. To help keep elevator clean of the wet dirt some guys have fabricated a couple plastic paddles (steel would also work) to install in place of rubber. I have not ever installed brushes onto auger but have heard that works well to keep the auger tube clean of dirt buildup. I'm glad we don't get this wet dirt deal very often.
When you pull up one of those stalks with 2 fingers check the ground under the roots and I will bet you will find grubs from the Jap beetles.Then find a stalk that is strong and pull it up and check for grubs.If there are any they will be on the edges not up in the root ball.Have had the same problem a few years and found the same thing every time and this year is one of them.I would like to know what you find.