The plastic in the Cali fuel cap has a tendency to swell over time, at best making it difficult to unlock, and at worst leaving you stranded, unable to put fuel into the tank. This happened to my Stone and I used the following procedure to fix it.

Take note of the relationship of the pin and other parts by putting a mark on the pieces. Remove the pin from the cap and disassemble it.

Using a big bastard, file the surface of the plastic where it meets the alloy. This plastic is damn hard, so spend a good 10 minutes or so at it so you don't have to do it again.

Reassemble and check it works. If it doesn't, you've inserted the pin 90 degrees out of whack, so pull it apart and do it again.

(Photos were taken by John Ulrich and posted on the Wildguzzi forum.)