There are a couple of reasons student screens might not be visible. We would want to narrow it down to ensure we can get those screens up and running!
To ensure it is not a refresh that is needed, first please clear the cache on the Teacher's device. CTRL + SHIFT + R.
If it is only a few students, excluding them and including them again sometimes does the trick.
If the above does not work, please see one of the specific articles depending on the message that appears for the student tile.