I'm thinking FC363 is right, you need a tool to put a small chamfer on the inside of the case neck.
Link here

The neck is sized as a straight cylinder and it has to be slightly smaller than the bullet so that neck tension will hold the bullets in place.
The de-burr tool shaves off the square shoulder inside the neck and lets the bullet start.
Check your brass for a sharp corner on the outside of the brass as well and notice the tools for removing it.

"Camping places fix themselves in your mind as if you had spent long periods of your life in them.
You will remember a curve of your wagon track in the grass of the plain like the features of a friend."
Isak Dinesen