Shared Workbook giving “read-only” message when saved

My department is using a shared workbook for tracking department data, and some of our users in the department work remotely through a VPN. One of these remote users is having a problem saving to the shared file. The workbook opens, and he can write to it, but when he goes to save, the file doesn’t allow him to save to the shared file, but claims it’s a read-only file. Other remote users have not had this problem. I’m looking for any ideas on what could be the trouble here (I’m running out of troubleshooting ideas!)