I can't tell what the issue is here but there a few things you can try to understand where it comes from.
Do you see user information on other views?
Make sure the settings for this table haven't been changed and it still sorts users by highest traffic. (can also try to return the view to default if you hadn't made changes to the view that you want to keep).
Search for the Application Control logs for example and see if you have logs with users in them and see if they have accounting section. If not, make sure the Access Control policy rule they use have accounting on it.
You can try to drill down the total traffic/users and see additional information that might help.
Kind regards, Amir Senn