"Houston, we have a problem"

[Rapidrob]

I've been reloading since 1963. What you describe is either a die not bored properly or your brass is too thick at the neck. I have yet to see a brand of dies that does not have a problem from time to time.
The most common problem is the brass. Remove the decapper/neck expanded and size a fired shell. What is the neck diameter after sizing? ( inside/outside)
Back your die off three full turns. Size a fired shell. Can you see a very distinct ring where the neck has been sized? Is it measuring too small on the inside?
Measure your bullets. Are they .264? ( or close)