A couple of things can cause this. One would be that some of your brass is longer than the rest. Any that are longer will go farther up the expander, and thus expand more, causing the problem your having. The next thing would be if the brass is thicker than the others, in which case it would, again, be moved more by the expander.
Check your brass. Anything headstamped A-Merc is junk and should be thrown away anyway. After that, trim your cases (Lee makes an inexpensive and easy tool to do this with). If you still have a problem, sort your cases by headstamp and readjust your expander die each time you change headstamps (which you probably won't have to do if you pitch all the A-Merc garbage and trim the rest).
There might be some other things that can cause this, but those are the two I would check first.