I've had the same issue with Instagram insights being blank when trying to see who shared my post on their story. It's pretty frustrating, but I've found that if you have a business or creator account, you should be able to see the story shares in your insights, but sometimes it just doesn't work. One thing you can try is checking your notifications, as you'll often get a notification when someone shares your post on their story, and from there you can see who it was. Not a perfect solution, but it's better than nothing.