Well, on mine, it was the centre pin. But mine is a metal quick disconnect. Yours looks like plastic in that photo...a smacking may not be the best idea
I have a water heater but my shower quit working a few years ago. I took the valve off and disassembled it and found check valves inside that basically worked to block any cross connection from hot side to cold. While taking it apart I lost a spring so I removed the other spring and reassembled the valve and have not had a problem since
Have you found your issue? if you can access the back side of the outdoor shower unit from the inside it should be a pretty easy test to see if the gaskets are stuck on your valves like ours were.
I sent Stan an email and was waiting his response before digging into it further. That will require shutting water off and draining the tank and I've got a full plate with work this week trying to get ready to go on a little weekend camping trip starting Friday morning. We can make it through the weekend without a functioning cold-water-only outdoor shower
I called FWC and they said it sounded like a bad valve so I purchased a new valve setup.
Finally got around to tearing everything apart this morning and the problem was a stuck fitting on the cold side of the valve. A little push with the Allen key and reassemble everything and now we are good to go.
Now have a brand new shower valve on hand if anyone needs it!