Are you head spaced too short?
It is common for brass to grow before the shoulder makes contact. If you chamber is shorter than your die to holder then the shoulder will not bump. You could possibly need to trim the die or holder or correct the headspace if needed. There is not a lot of room for error, stranger things have happened.
Brass spring back is also a possibility or a portion of the issue..
How many times has the brass been fired. Do you anneal?
What cases are you using? (270 Win for example)
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