I know this is an old topic, but I came across it by doing a Google search. I just had the exact same problem happen to me. It seemed that randomly I would get a case that filled with powder and overflowed all over the press. I contacted Dillon and they were great.
I finally figured out that one of my cases had dirt caked to the inside wall. When this case got to station #2 (powder), it would overflow. I dumped the powder out of the case and threw it back into my pile of empty cases. When this particular case came up to get loaded, the same powder overflow would happen.
This drove me crazy for about a week. I thought something was wrong with my press. It was that the case had junk down inside it so a full powder charge wouldn't fit....causing the powder to spill all over.
Hope this helps somebody in the future.
