I received a Mossberg 590 with ghost ring sights for Christmas. Lucky me! <G>
While inspecting and testing it, I noticed that if I load only ONE shell in the tube and then rack the action, roughly 70% of the time the shell just sits in the tube and won't drop onto the elevator. If I load a second shell and rack, THAT shell will load fine, eject and the remaining shell will, again, stay in the tube. Manually depressing the cartridge stop allows the stubborn shell to pop out just fine.
I then field stripped the weapon, cleaned and re-lubed with CLP and Tetra grease and reassembled.
My first thought is that the mag spring may be a little weak. Either that, or it doesn't like the Estate shells that I have.