Looking at the ring on the bullet it appears you may be using a RN seater plug in the seating die?
Don't know about all 40 dies or what your using but most have a TC seater plug, I know RCBS does.
If the bullet is even slightly misaligned when its seated you can get a little case bulge. In the tight chambers thats sometimes just enough to cause problems.
Try measuring a loaded round that doesnt fit right about the depth of the bullet and keep turning it to see if you get the same measurements all the way around.
Thats just a guess because you say you getting some rounds that wont go 1/2 way, sounds like you gettin some that do fit?