A couple of questions:
You said the do-nut is at the junction of the case body and shoulder. Normally a do-nut is on the inside of the case and is at the junction of the shoulder and neck
and obstructs the passage of a bullet below that point on a fired case. Is this the do-nut you are talking about? If not, can you post a pic? I'd like to see one at the case body/shoulder junction.
Second question would be how are you setting up your dies? i.e. are you threading them in till they hit the shell holder, a little further or a little less?