 Why does Adam refuse to play checkers?
But this might be so varied or so unfamiliar that often both the machine and the designer of it were puzzled as to how to read the results. But when the Locator had refused to read Positive when turned on the inventor himself, bad blood developed between machine and man. It was for this reason that Glaser used that model no more, but built others more amenable. On Earth it had read Positive on a number of crack-pots, including Waxey Sax, a jazz tootler who could not even read music. "Not a building, not two sticks or stones placed together. And those two ... why, they could well be Earth-people.
 He is scared of losing and giving away his true identity.
 He does not want to humiliate the priest by beating him.  The priest is too eager to go up against him, and he doesn't want to disappoint.  He has no reason to play. He is omniscient and would win without contest.  He is scared of losing and giving away his true identity.