Students not appearing in OneNote

Hi all. I have a colleague who seems to have students missing from her notebook. Now they have access to her OneNote, however their personal folders are not visible to her. When we try and add/remove students the student is not listed. When we try and add them, it then says that a student with the same name is already a member of the group and we cannot proceed. I am quite handy with all things "Class Notebook" however I have not seen this before. And to top it off, it is happening with all her notebooks. 

Any suggestions? With remote learning in place, she is unable to view the work they are uploading to their folder. I am not sure whether the student themselves can see their folder. 

Thanks

Nick