[Billhilly]

I still like the rubber mallet idea, but as you've already tried that.....
I had a different situation where I couldn't get the cap the latch on, got a second slightly different one and had the same problem and ended up smacking it with my fist. That did it! Then I couldn't get it off! Turns out the lip just inside the filler neck was rolled away and the locking tabs had nowhere to lock. I ended up using a slide hammer to bring things back square inside the filler neck. It's possible the inside edge has been damaged (pump nozzle?) and now's it's caught/bound up. Bigger rubber hammer? The key operated ears on my one are very positive. When you turn the key you can't hold them. So unless there was a complete breakdown internally, if the key is turning and springing back is indicating it 'should' be functioning properly. Weird.