There could many reasons. The brass may have soft/hard spots causing the FMJ bullet to not be centered. Happened to me many times. The FCD will solve that problem and all rounds fit the cartridge gage. Many shooters/reloaders on this forum uses the FCD on FMJ bullets to make sure that they get the best in feed reliabilty. It does work, and well.
The issue with lead that it will size down the bullet with a smaller diameter causing accuracy issues, but not all the time. It does come and go, depending upon the bullet, the diameter, the brass, etc. I have shot lead bullets with great accuracy and have seen it then start spreading out, then going back to being accurate. Again, with lead bullets, it will depend upon the diameter of the sizing ring in the crimp die. You can order one from Lee with aonther thousands in diameter for use with lead bullets. Again, a friend did that it worked well.
Regards.