I had watched Tasogare Otome × Amnesia quite a while ago. It was never clear to me why Momoe Okonogi couldn't see Yuuko.
I take it that to see Yuuko you needed to
Clearly Okonogi believes in all paranormal things, and she is always talking about Yuuko. It is also later confirmed to her that Yuuko indeed exists and is around the club.
Why can't she see Yuuko?
As Yuuko explains, people see her how they believe she'll look, it could be that Okonogi believes ghosts exist but can't be seen unless they're using a medium like with the doll Teiichi exorcised which would be backed up when the "evil spirit" disappeared.
Given that since Teiichi to her is a spirit medium she is probably expecting Yuuko to be invisible and use Teiichi to speak to everyone which would explain why she can't even hear her.
