Thanks but everything is tight. The switch is bad.
I'm not trying to cast doubt on your abilities here but, overlooking the retaining ring is such a easy thing to do. Just to double, double check, you unscrewed the tailcap and used a pair of needle nose pliers to tighten down the thing silver ring with two holes in the bottom, right? (If you already have then I would disassemble the tailcap, gently clean it, and then reassemble it.)
If none of that works then you will have to warranty it. Fenix usually does their warranty through the dealer you bought the light from. You will want to contact whoever you bought the light from and have them talk to Fenix. You will need the warranty card that came with the light and the light itself to be sent off. Hopefully that will get you some better results than you are getting now.