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What model is your MEC. Is a signal stage or a progressive and does it have any shells in it. I think if it has any shells in it the die that does the resizing the shell that is in the sizing die is way out of round and it won't let it go back up. I have seen that happen before. it has been known to happen if the shells come fro two different guns. I hope this helps but with out seeing the press and having it here to look at all I can do is tell what I know has happened before.
I did not see that you had put the model down and year. I have seen a 9000 before were a guy had bought it used from some one and he had not set it up right as he did not have the original owners book. He had to contact MEC to get one.