I believe most of the "tumblers" like this activate electromagnets in sequence to create the vibratory action.
Mine works that way, but you left out a step.....
If it uses an electric motor then yes, it uses electromagnets in sequence to spin the motor. But electric motors spin pretty smooth, and don't vibrate much.
It is the spinning shaft of the motor that powers the vibrating action, by spinning a flywheel disc that is "lopsided". Meaning the wheel is intentionally out of balance, heavier on one side than the other. It may not even be a complete wheel, but simply a bar heavier on one end than the other. Take a look at your tumbler from underneath, you'll probably see it
spinning the unbalanced wheel creates wobble. This wobble shakes the bowl, which is mounted on springs to allow it to shake. That is the vibration of a vibratory cleaner.
Ike, you need more media, fill the bowl up to about the top of the "dirt ring" (after you clean it off) on the center spindle. Also, remove the bottle necked case I see at 12o'clock in the picture. Never tumble cases of different sizes together, if one case can fit inside another, it will, and it will get jammed in there by the media.
More media allows a deeper "sea" for the brass to "float" in What you're seeing is your brass "washed up against the cliff" (bowl wall) and not enough media to "wash them off and suck them back out to sea" so they get stuck there, and base down because that's the heavy end of the case.