I find it intersting that Kynoch brass fits perfectly, yet new brass doesn't. Should be able to figure it out. This means there is a different measurement somewhere that needs to be found and solved.

There are a number of normal reasons one might fit and another not fit.

Can you see on the case where they are binding with bolt closure?

Shoulder large?

Base large?

Base to shoulder measurement long?

Base to neck measurement long?

Your dies might not be capable of sizing the case sufficiently to allow easy chambering. This is a common problem with some factory and lots of wildcat dies made to 'other specs', & not made to the case fired from the rifle. This happens with one custom die maker, at times.

Are the necks well lubed inside before sizing? If not, the 'drag' friction of pulling the expander button out throught the neck can easily pull the shoulder foreward, making subsequent chambering difficult - each time.

Overall length (possible long necks) is another factor - does a bare bullet drop into a fired case easily or does it bind at the end of the neck?