Calling SureFire is a good idea.
However, you may like to show us a photo of your G2 TailCap because we may be able to help.
When you remove the TailCap does the metal switch contact assembly (the component with the spring attached) fall out or is it fixed inside?
If it is fixed, it should jiggle around a bit and you should be able to rotate it easily relative to the TailCap housing. Also, you should be able to push/pull it backwards and forwards about a millimetre of travel (this is what should happen when you press the rubber button).
When you press the button you are overcoming the spring contact and moving the switch contact forward so that it makes contact with the metal sleeve of the G2's body. If the switch contact assembly is so-how jammed inside the TailCap or not correctly seated inside the TailCap it will not function correctly.
Also, if the contact surfaces are not clean this can lead to poor quality contact and function.
Thirdly, it could be that the Lamp Assembly spring contacts have been somehow compressed so that (usually the outer spring contact) does not make reliable contact with the metal sleeve inside the G2's body. You may care to carefully stretch the spring contacts slightly (don't pull the spring from the Lamp Assembly - stretch a portion of the spring instead).
Please let us know how you get on either with SureFire or with any of my suggestions.
Cheers,
Al