On the end of the screw is a 90 degree cantilever arm. Turning to tighten rotates the arm to engage the inside of the panel to which the outlet is secured. Further screw tightening locks down the arm to the inside face of the panel.
I've found that I can usually push real hard on the screw first, and then rotate it CCW 1/4 turn to rotate the arm free of the wood. You may have one of them disengaged, or they may be close to disengaged. You'll just have to fiddle w/it gingerly as to rotation of the screw but w/a heavy hand as to pushing in on the screw.
During construction they may have used a zip-saw to cut the hole for the receptacle. Zip-saws leave a raggedy edge in cheap paneling, so the little arm may be hanging up on some wood fiber on the inside edge of panel.
Worst case- you drop the little arm inside the wall, and have to get a new outlet at Allied Trailer Supply, El Camino Ave, Sacramento (916-922-3487) next time you head to town to do big shopping. I'm in Auburn, & they are my go-to parts place for stuff like that. I believe you could email them a photo of the item, maybe w/a tape measure in the photo for better scale, and they could ship the gizmo to your house
alliedtrailersupply@yahoo.com