This issue can occasionally come up, and the most likely cause is either a slow internet connection or trying to share a large file so it can take a while to load.
Here's what to do if the file won't load at all.
The fastest fix for when the student can't see what you see is for both teacher and student to refresh their whiteboards. You can do this by clicking on the little refresh button on the top left of the whiteboard.
When you refresh your whiteboard in this way, all of your annotations will be saved.
If this doesn't work, you may need to close the tab and try to reload the material again. In this case though, your annotations will not be saved. It might be worth asking your student to redo their equipment checks before trying this. Re-entering the class might solve the problem, and you would avoid losing your work!