Anyone on here good with plumbing?

My shower has a pretty steady drip out of the faucet. I've replaced the faucet itself and the cartridge where the handle turns on. No dice.

Today, I took it all apart and cleaned all the surfaces that mate together, and rubbed Vaseline into the O rings. It did not solve it. I watched YouTube videos on the subject and they all say to replace the cartridge.

Is there some reason it could bey shower head letting in air to the line or something? It's the detachable kind and I took it off the hook which temporarily stopped the dripping. We have pretty hard water, and the shower head has some calcium buildup.

What's my next move?