Is your seater die adjusted properly? My brother once messed around with my 22-250 dies and my seater die would crush the shoulder of my cases and prevent them from chambering. I turned the die out a few turns and the problem went away. I felt really stupid once I had figured out the problem but I didn't know he "adjusted" them.
Also, did you check your case length? If it was too long it could cause problems chambering.