It is possible to tighten up the neck on a case by simply driving the case a little deeper into the resizing die. Take a dowel that is no larger in diameter than the rim of the cartridge, and drive the case into the die just below flush. This will further reduce the case mouth diameter and hold your bullets a little tighter.
There may be a variation in how tight bullets are held in place. This is due to the case being sized from the outside of the case. If the case has a thin wall, the inside diameter ends up being a little larger, than if the case has a thick wall. Sort your cases, and resize accordingly.
