You aren't really giving us enough information.
What's your equipment?
What components are you using?
New brass, once fired, range brass? What head-stamp?
What projectiles are you using - make? materials? castings, plated, manufacturer?
Have you taken calipers to your brass and to the projectiles?
When I have this issue - it's usually on a piece of brass that's been thru my bench 5 times or more and is ready to be retired to the scrap bucket. OR - a cast round that's below the required dimension.
.40 S&W brass is straight sidewall on the outside but has a very slight inward taper on the inside of the case. So a bullet shouldn't simply slide all the way into the case unless the brass or the projectile is out of spec.