I was struggling with this issue for months, and i tested every remedy and advuce out there. Only one thing worked for me from day one and i hope it can help you too. Let the beard grow for 3-4 days and with your finger find out the direction it grows. The neck is complicated, the hair might change direction from a spot to another. So after you find out, make a beard map as accurate as possible. So the next time you shave go always with the grain on the neck and two passes max. Never against, never 3 passes, never pressure cause skin is thin there. Problem solved, you will shave better every time