Wizard,
It's a common misunderstanding that resizing in a full-length die returns fired brass to its original, unfired dimensions; it doesn't. Small base dies will take these down to closer to the original dimensions, and might be worth a try here. Once the brass chambers easily and has been fired in your chamber, it should react well to the conventional full length sizing (or even neck sizing, if you chose to go that route). This is more than likely a case of brass that was fired in a somewhat more "generous" chamber the first time around, yours being a somewhat tighter chamber, or a combination of both.
Fairly common situation if you insist on using brass fired in other chambers, which might make a small base die a worthwhile investment for you.