Seems that this is not uncommon, per a previous thread. One suggestion was to remove the locking cap altogether. I tried this on my Roadster 3.7 and the car ran without any problems. There is a suggestion that the locking cap is being held in by vacuum.
It is possible that it is swelling due to interaction with Ethanol, my KTM filler cap was affected by this and needed a light sanding every so often.
