How long ago did you neuter him? I think I remember being told it can take up to a month. If it's been longer, have you considered getting a professional trainer in to assess what's going on?
Just so you know though, we neutered both my males last December. Prior to the neutering, neither was aggressive. Well my older one (he's two) now decided he doesn't like intact dogs. He's gone after them. This was never an issue before. So I'm wondering if that's what's going on with Apollo and your girl.