She might have a very good reason for not wanting to wear one. Have you asked her?

She might just have some misconceptions about having her vital organs squeezed or relocated. She might have some concerns about health risks. You need to ask her why she doesn't want to wear one.

And since Mobius' suggestion is a subtle form of manipulation, I have to disagree with what he's said, which is a rare day indeed. Just trying to get her to do it by slowly introducing it and not talking to her is a way of manipulating her to do what you want.

Now, Stop talking to us and go talk to her.

Have a great day!