Not displaying shared users

Not displaying shared users

For the most part all of my users that I have shared the sheet with are displayed when I 'Edit Shared Details', HOWEVER one of my colleagues that I recently shared with is not displayed. Thus, I cannot edit their permissions or which sheets they have access to (for some reason they have access to everything which is a big problem). 

Please help as soon as possible.
    • Topic Participants

    • Dan