I assume you are using different Developer Accounts with different App ID's. The Game Center Game is tied to a specific Bundle Identifier which consists of the Bundle Seed ID and your Wildcard App ID. His provisioning profile for his Bundle would have a different Bundle Seed ID and possibly a different App ID which results in Game Center not recognising the game for your Tester.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.