[NIGHTHAWK]

Hello, 1999 Bird. Key will turn all the way when cap is open, but will not turn far enough to lock and remove key when closed? Any suggestions? Thanks.

 

[Punts]

Sounds like the two latches or pins are not locating fully.
Check that there is nothing stuck in the two holes in the part screwed to the tank. anything in the holes might stop the pins from returning fully.
Also check for any "step" or wear on the pins.
Sometimes on mime, I have to push down on the cap to locate the pins and get the key to return.

 

[Aussie Dave]

Yep, push down harder and the key should 'snap' closed by itself

Maybe try some WD40 or CRC spray lube to free up any sticky bits?