In soils that are wet, none of the chain type potato diggers sift well. However by using draft control, I do not see that changing. Draft control will just manage the depth of the soil, not the soil going through the sifters. What is the spacing on the chain? Mine are set up for one inch potatoes so that we can cater to the early markets, it does get to be a pain later in the season when you no longer need the small ones.
Mine does not handle the damp soils well either, the clumps do not sift through either and you have a lot of dirt going over the top with the spuds. Tractor speed also plays a big factor in the amount of the dirt being sifted through. When the soil is damp, draft control will be of no use to cure this problem, unless you are not maintaining a steady depth.