Co-Author spreadsheet - Read Only

We have a small office that has three Authors to the spreadsheet. It's been working out great for years, and now (as of yesterday) it allows the first person to open the spreadsheet to do the edits but the other two can only open it as a Read Only file. I have tried to re-save in a different location, re-add the authors, and of course get into setting and allow all users on our network access. All to no avail. Why is co-authors now only able to open in Ready Only?

Please help!