A: How are you choosing to define normal? If you get back to someone right away or within a few hours, you have decided that’s the “normal” response, but your friend may think “normal” is within a few days. Her version isn’t a deviation of normal, it’s her normal.
It’s entirely possible that she didn’t think the text required an acknowledgement or a response, or she meant to respond and got distracted or she typed a response and never hit send (I do this all the time).
Pay attention to what you deem normal and more importantly investigate what you are making her slow or lack of response mean about her, you and your relationship. What if neither of you are wrong and need to change? What if your friend's perspective is perfectly normal?