As these caps age, many become difficult to open. I am guessing that the fuel vapors dissolve any lubricants in the lock and perhaps some corrosion sets in. If all else fails, you can remove the cap and ring by unscrewing the setscrews, and open the cap from below by carefully prying back the 'fingers'. I would then try using a dry lubricant like powdered graphite - squirt a little in the lock and then work the key back and forth. Alternatively, you can try a product called Lock-eze, which seems to be graphite in a very volatile solvent. The liquid evaporates leaving the graphite behind. Since it is in an aerosol can, this can be quite messy to use.
After you finish playing with it, you might consider one of the aftermarket screw in caps. I have an Oberon, but there are others that are cheaper.