The current explanation for the Devil Spell or whatever is that no matter how awesome their logic is, they can't quite piece it together. So they know he unmasked, but can't remember his face. Norman probably knows that he knew it before, and he knows that he killed Gwen, and he might even know that he killed Gwen to piss off Spider-Man, and it's not impossible that he knows that Gwen was Peter's first love and the only one he'd never want to live without, but no matter how he approached the problem, he can't make that tiny jump to Peter being Spidey.
I fully expect some author to come along and make everything I said into vile lies, but I think they're true now.