The usual source of such problems is insufficient sizing resulting from a failure to compensate for press flex under load. Examine die/shellholder gap when sizing a case. If gap is present, die can be adjusted farther down.

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If gap under load is eliminated and cases still will not chamber, upper surface of shellholder can be stoned or sanded thinner to allow case to enter die farther.

Before trying any of this, measure rim thickness of problem cases. If it's greater than .064" (and your rifle has good, tight headspace) the problem may not be at the shoulder.