Neck of casing flat spot?

[RStewart]

I'm assuming you mean the mouth of the neck, not the whole neck?

If so, its usually caused by the case mouth striking the brass deflector, or by hitting concrete when it hits the ground. If it is every case, its almost always the brass deflector on the upper, and can be reduced by putting a small piece of the fuzzy side of velcro on the brass deflector.

^^^^this.