This could be for a number of reasons, but your post doesn’t give enough information for me to say definitively.
If the bulk update is returning with a message of something like “73,000 user author slugs updated,” this is to be expected. There are many factors that could result in this, but ultimately, the plugin will skip updating a user that won’t, or can’t, be updated.
If you’re getting a PHP error or a server timeout error, I have a potential fix coming for this soon. However, there are no guarantees the fix will allow you to update all your users. Again, there are a lot of factors that could lead to these errors. Unfortunately, updating them all may not be as simple as clicking a few buttons.
