It could mean they've lost interest. Maybe they started seeing someone else or just realized they don't want to pursue anything with you. For example, they might have met someone new who takes up all their attention.
If it really bothers you, you might consider having an open and honest conversation with them. But be careful how you approach it. Say something like 'I noticed you haven't been looking at my Insta stories lately. I hope everything is okay.' This shows that you're concerned but not accusatory.
When your ex stops watching your stories, it could imply that they have emotionally distanced themselves from you. It might be that they don't want to have any reminders of the past or they're consciously avoiding any interaction related to you. However, it's also possible that they just got busy or lost interest in social media in general.
It could indicate several things. Maybe she's got a lot on her plate and doesn't have the time. Or it could suggest that the content of your stories isn't resonating with her anymore. Maybe there was a misunderstanding or something you posted that rubbed her the wrong way.