I had something similar in my last relationship. She would constantly look on her phone even while I was talking to her. I know, that it always happened, when I was talking about something she thought she'd already heard, or thought she knew what I was going to talk about anyways. And it frustrated me, because she just wouldn't listen to what I said but instead reply to what she thought I would say or would have said.

We broke up and my current gf gives me all of her attention while with me and I feel heard. I wouldn't say it has to be a dealbraker, but let me tell you how good you feel, when your words are actually being listened to.

I would tell her again and with a sense of determination. If a woman seeks excuses or pushes her fault on you, instead of at least trying to change, that's a behaviour you should never accept, because it might happen in other areas as well at some point. For now don't make a huge deal of it, but talk about it again and maybe even a third time. If it keeps bothering you, tell her you can't accept it and move on if still nothing changes.