I have a Ruger #1 with a custom 257 Wby bbl. It shoots very well , but I have a problem when I neck size only using an RCBS Neck Sizer die. I adjusted this die to size the neck fully.

The problem is that the loaded rounds are very difficult and sometimes too hard to chamber. I have tried using the RCBS Full Length die adjusted to neck size about 2/3 of the neck and this works well.
When I examine the rounds that failed to chamber I notice some scuffing around the case just behind the junction of the shoulder and the case body. I don't really have a problem with using the FL die but It seem that the neck sizer should work?
Any thoughts appreciated,
Thank you