It would be dirt, or the cartridge, but I've only seen that problem with reloads not factory new rounds.
I think you mean what is usually called the recoil shield. If the cartridge is riding high out the back of the cylinder it will bind with the recoil shield. The recoil shield itself isn't likely the problem as that area is just very unlikely to ever incur damage. It's a problem with one of the cylinder holes not allowing a fully inserted cartridge for some reason or a damaged cartridge not seating all the way in the hole.
Right. That's what I meant... the yoke (or whatever the star-shaped part of the ejector is called) is seating flush with the cylinder (nothing underneath it between it and the cylinder) but the cartridge rim isn't seating all the way against it, causing the cartridge to sit higher than the other rounds and binding against the recoil shield.