I had trouble recently as well, but I had dipped x 2 so it would slide loosely
it was the devil removing it! I tried putting it in the freezer etc but finally let it sit in water with vinegar for a few days. At last it came loose and cleaned up well. I put a sealant on the key (due to the acidity of the vinegar to the stainless steel key) after a light scrub of steel wool.
Good luck, and have fun! I
I was not giving up, so don't you!