If you are able to achieve a close shave there via a DE razor or some other razor, I suggest that you pay close attention to how that blade is coming into contact with your beard vs. how your straight razor is. Up and down might not be the best way for you to do that. I will tell you what I do only to demonstrate that its unconventional. There's no reason to think that what I'm doing will help since my beard may be completely different than yours. But my first two passes on the neck are kind of up and down but more in an X pattern. From right side mid way between the chin and the ear down to the left middle at the base of the neck. Then up from the right middle base of the neck to the upper jaw midway between the chin and the ear. The final pass is directly side to side, right to left. I do this because this is how I was doing things successfully when I was using a DE. Same shave, different razor.