90 yo mother doesn't respond at all at times. Is this normal?

Why don't you ask your mother? She may be confused by the question, but it's a perfectly reasonable question to ask. You don't have to be sarcastic or defensive - e.g. "am I boring you, mother?" "Hello? Earth to Mother?" - but simply say that you've noticed she doesn't seem to have anything to say about the grandkids or your own news, and you've often wondered why.

She probably just can't keep up with the storyline, you know. Does she get plenty of up-to-date pictures?

Have you ever thought that maybe your mother is hard of hearing and might need hearing aides? That is why she is able to tell you all what's going on with her, but doesn't respond when you tell her things, as she's not hearing you properly. I mean she is 90. It could be something that simple.
And if that is not it, then I would just ask her why she's ignoring you. Wouldn't you rather hear it from the horses mouth, then be second guessing what the issue is. I wish you well in getting this figured out.

She may be chatty because she is in control of the conversation.
When it comes to RESPONDING to questions or commenting about your conversation there might be several things going on.
Is it possible that she can not hear you correctly or clearly?
Does she realize who you are talking about? If she is not sure she may not want you to realize it or be embarrassed that she does not recall.
Talk slowly, distinctly and use peoples names and how they relate to her.
"Your grand daughter Becky hit a grand slam in her ball game today"
"Your grandson Stephen is going on a canoe trip"
Keep conversations short so they are not exhausting.
Give her plenty of time to respond.

The "little memory loss" might be more extensive than you realize. Think back over the past year, or two or three and put together all the little things that you might have brushed off or she brushed off and see if it adds up to more than a "little" and if this is something that should be addressed. People can "hide, cover up and work around" dementia for sometimes 10 years before it reaches a point where it is noticeable.

Thank you for your responses. I talked with my brother, who sees her more regularly than I do, and he thinks she may have a bit of a hearing issue.

I primarily talk with her on the phone as I live several hours away. I don't think hearing loss is the reason she doesn't respond as she seems to have no trouble understanding my questions and comments about what she says. I guess I've just felt hurt about it and didn't want to hear her say anything negative (which is her tendency) so try to keep the conversation light and flowing on topics she's interested in. However, I will ask her outright why she doesn't respond.

It's funny, but yesterday when I talked with her she chatted away about Facebook posts of my former sister-in-law (whom she never interacts with and I don't keep in touch with) and how happy she seems to be in her new location, but only said "oh" when I mentioned I was going on an upcoming vacation to an interesting destination. I tried telling her a little about it and again, crickets. It seemed odd to me.

A month ago I mentioned that her grandson and his fiancé were finally starting to plan their wedding (on hold because of COVID), a subject I thought surely she would be interested in and once more there was no response.

This all doesn't add up to me and I have to say I find it difficult to call her. But I will follow up as suggested. Thanks.