the lee loader is a neck sizer only. since you mention this is for a mosin nagant i have a question or 2. first, have you shot this brass in your rifle before? i have one nagant and have experience with a few others. i had the same type of problem as you do with one of mine. the rims on the brass were too thick for the extractor to snap over, it is a common problem with there rifles as i have found out. on mine i couldnt lock the bolt closed without a rubber mallet. unfortunately rim thickness from one brand to another is not consistent. if you have shot this brass before then there should be no problems, but if this is the case i would suggest you get a full length sizer die to negate this problem. second, do you have the bulets seated far enough so you are not jamming them into the lands? that could also be the problem.
ron