2nd story shower stall (fiberglass) when the shower is running there will be a small leak downstairs directly under the shower coming from the floor boards. I've taken off the shower drain faceplate and counterclockwise tightened the pvc drain. This didn't help. I will say that the face plate has ZERO caulk underneath or above on it. Could this be my small leak issue? And it only leaks when the shower is running so I don't think it's an internal issue or I would have water consistently. Or could it be the drain further down below the flooring?

And it only leaks when the shower is running so I don't think it's an internal issue

That is not necessarily correct.

You could have a leak at the drop ell, where the shower arm screws into the shower riser pipe.

Pull the escutcheon around the shower arm away from the wall and with a flash light look inside the wall for signs of water.

The ultimate test of that joint is to get a 1/2" PVC threaded pipe cap, remove the shower head and screw the pipe cap onto the shower arm. Turn on the shower and look for leaks.

The reason I said it only happens when it's running is that the floor joist and base floor underneath are dry where I normally see it when the shower isn't running. I'll head to the hardware store and try pvc pipe cap. Thanks. If no signs appear. I guess I will run caulk under and around the shower drain to see if that's where it could be.

You put your finger directly on the problem when you said,
I will say that the face plate has ZERO caulk underneath
You have lost the drain seal.
A drain seal can be ruptured/lost if the shower floor isn't bedded in at installation and gives and flexes when you step into it. Purchase either a can of Silicon Gel or Plumbers Putty. Unscrew the top of the drain and make a "rope" of putty or gel and place it under the drain lip. Now tighten the drain lip clockwise until tight. Remove the excess that got squeezed out and the leak is gone.
Be aware that if the shower floor still gives under weight that this might happen again. Good luck, Tom

Tom, so when you say tighten the "drain lip" your not referring to the drain plate with the two screws? Sorry, I just want to be sure.

Just to test for a bad seal try filling a large bucket of water at another faucet. Dump the bucket slowly into your shower pan. If you get a leak then it's the drain seal and not part of the mixing valve or shower arm.

when you say tighten the "drain lip" your not referring to the drain plate with the two screws
No! That's called
"the strainer,( see images) Remove the strainer and unscrew the top part of the shower drain and follow the directions I already laid out. You can test for a ruptured seal like Bob posted but it's clear to me the seal's gone. And always remember that the rubber gasket goes UNDER the shower floor and not beneath the lip. Good luck, Tom

Well, I slowly poured about 7 gallons of water down the drain slowly and no leak downstairs. Then I put a 1/2 cap on the shower head pipe and heard dripping inside the wall and looked into the hole and saw it "spitting" out of the connection from the shower head pipe and the copper lead pipe. Purchased new piping, put some dope on it and we'll see if that's it... Hopefully there isn't something further down inside the wall that I can't get too..

Thanks for all your help. I appreciate it.

We thank you for the update. I don't think it's farther down or you would have water leaking 24/7.
Let us know if that fixed it. Good luck, Tom

Glad you did the test, proved the drain wasn't the issue. Not really sure what you did here "copper lead pipe" but if its dry then you are good for now. I'd check this patch in a few days to see if its still dry. Wrap the mixing valve and patch with toilet papper, if its wet later you will know you still have a small issue.

Bob,

I think he means the shower riser pipe. "Lead" like in like "Bob is a great leader at AMHD. Vote for Bob for president". Not "lead" like in lead poisoning.

Thanks Harold. Some folks say I lead like lead. Get it?