Fit Problem 10: Gap Behind the Neck Despite Tight Collar

The Problem
When your dress shirt is fully buttoned up, the front of the collar is pushing against the front of your neck uncomfortably but there is also a gap between the back of your neck and the back of the collar.
This is likely caused by you having a posture where your neck goes slightly forward and your shoulders going slightly backward.

The Solution
First and foremost, try to stand in a relaxed posture when you have your measurements taken. Secondly, to relieve the tightness at the front of the collar, you can make the collar larger.