It could mean she's just casually interested in your content. Maybe she doesn't want to commit to following you but still enjoys seeing what you post from time to time.
There's also a possibility that she has some kind of personal reason not to follow. Maybe she has a full list already, or she doesn't want others who follow her to see that she's following you. But she still likes to check out your story when she has the chance, kind of like a secret interest without the commitment of following.
It might be that she is shy about following you. She could be interested in you or what you post, but for some reason, she doesn't feel comfortable with the idea of following. So, she just watches the stories as a way to keep in touch without making it too obvious. For example, she might be worried about what others would think if she followed you.
Maybe she's just casually interested in your story content without wanting to fully follow you. It could be that she enjoys the specific things you post in your story but doesn't want to commit to following your whole profile.
It could mean that they still have some level of curiosity about your life. Maybe they're not ready to completely let go, but also don't want to fully engage by following you. It's like they want a little peek into what you're up to without making it obvious that they're interested.
Maybe he's just casually interested in your stories without wanting to fully follow you. He might not want to seem too eager or committed by following.
There's a possibility that he's just casually browsing. He might not follow a lot of people but still checks out interesting stories when they pop up. Another reason could be that he doesn't want to be inundated with all of your posts in his feed if he were to follow you, but likes the quick and ephemeral nature of stories.
Perhaps she doesn't know what to say in response. Watching your stories is her way of staying updated on your life without having to come up with a reply. It could also be that she doesn't want to engage in a conversation directly but still wants to be part of your world through your stories.
Perhaps they don't want to clutter their follow list but still enjoy the quick updates from your stories. Another possibility is that they're a bit shy or private and don't want to be an obvious follower but still like to keep up with what you post.
Perhaps she just prefers to observe silently. She might be the type who likes to take in information passively, like through your stories, without the pressure of having to reply to a text. It could also be that she's in a situation where she can't text back at the moment, but has time to quickly check out your stories.
I don't think you should be overly concerned. It could be that she's just exploring different people's stories and yours caught her eye. There's no real indication that there's anything negative going on just from her not following but watching your story. In fact, she might eventually decide to follow you if she continues to be interested in your stories over time.
You could just ignore it. It's her choice not to follow, and if she enjoys your story, that's fine.