There are a couple of scenarios that could be preventing you from seeing your student's additions to a Google document. The more likely cause is Offline Editing.
If your student is on a mobile device, it's possible they opened the document while they were still connected to the internet. The document remains open on their device and they are able to see and even edit the document while offline. Any changes made to the document will not be saved, and visible, until they are connected to the internet - and yes this can happen!
If your student refreshes the page before connecting to the internet again any changes made will unfortunately be lost and the student will need to re-do the assignment.