Prompt to save changes not working
 

I have seen several people posting this problem however cannot find a
solution. I have an excel spreadsheet that multiple people access. Other
users, when they make a change to it are prompted to save the changes. Last
week, I was as well. However this week, when I open the spreadsheet, make a
change, then select "File" "Exit" (like I have always done and the others
users are as well), I am NOT prompted and (obviously) no changes I made are
saved. Does anyone have a solution as to why this is no longer working on my
system?