Trying to Understand My Friend

I(M) have a friend who doesn’t easily express her feelings. We talk every day, and it’s been three months since we became friends. She often has a poker face, which makes it even harder for me to understand what she’s feeling. I’m trying to understand her a little more each day, but she isn’t fully opening up to me. I know most INFJs can be like this, but she means a lot to me like family, and the only person I truly have that I can open up. I've tried talking to her about it, and while she has opened up a little, she doesn’t express much, whether she’s happy, sad, or upset. I often have to ask her repeatedly to share, and while I don’t mind doing that, I sometimes feel she’s still holding back. It’s making it harder for me to truly understand her. I just want to learn more about her and find a way to connect better.

Also what can i do to understand her more and more (I love this idiot)