She may not see it as masturbation. She may have been taught that it was just "scratching an itch". A lot of hyper religious people literally convince themselves of this in order to prevent eternal damnation due to sinning. She may feel intense shame. She may start to masturbate in her sleep, only waking when the big O hits. There are so many variables. Have you asked her to touch herself for your viewing pleasure? Maybe mutual masterbation will show her that it is, in fact masturbation? A lot of people feel intense shame while pleasuring themselves. If this is the case, only therapy can help this. Therapy and patience. Does she have past trauma surrounding sex? Maybe someone catching her when she was young, and telling her how horrible she was?