I suggest that talking with you was lower on her things to do list. I suggest that "I'm available on Friday" is not a firm commitment. Her give me a call is
Is letting you know that this is not a firm plan. In my world, my friends and I know that life happens and the call is checking in to see if the other person is still available. An unanswered call tells me she's not available now and if she didn't call me back I would know she's still not available. I might call again the next day but I'm more likely to get busy and not call back.
Vague plans are not for sure plans. As to not calling you, I suggest that she was not thinking that talking was urgent. She knew that you wanted to get together. She was busy and may have thought she'd call you when she had free time.She may be a much more casual, go with the flow sort of person while you take the vague words as a commitment. For me, if we haven't set a time, not getting a call back, just means getting together won't work.
Your post sounds like you had an urgent need to talk this weekend but you didn't tell her that. As a busy mom and grandmother, I wouldn't have given this a second thought because I would move on to other things that needed done. I would't have texted either. I would just call her again later. This is called phone tag.
My daughter has 4 kids, 2 of them special needs. She sometimes misses messages. Both of us often thinks if this is important they'll call back. For me, when someone asks me to call and don't give me any information, I believe calling is not urgent and will call when I have time.
This is a lengthy message to say that you don't know why she didn't call you back right away. I suggest that your communication was vague and she would have no way of knowing the get together was urgent. When one wants a specific thing to happen that that person has to be very clear about what they expect. I've learned to include in my remark Something like right now I think I'll be free. Give me a call (to find out). Because the two of you had not made specific plans I wouldn't have expected anything from her. I'd just try later.
I'm not always clear about plans, I do sometimes forget an appointment or.plan, and am in many ways not perfect I give the other person the benefit of doubt and hope they do The same for me.
In response to your comment about a 5 second text indicates to me that your life is calmer and Better organized than my daughter's. For example, just as I get a phone call, 2 of my grandchildren are running around crazy while I'm trying to get dinner on the table. The TV is too loud so sometimes I don't even hear the phone ring. I don't look for messages until I get home several hours later or sometimes the next day. Perhaps you carry your phone with you most of the time. My daughter doesn't.I know that if I need a response right away, I call. She usually hears her phone but sometimes she doesn't. She may not see a text until the next day.