ms excel courses - "Read-Only"

Sometimes when I open the Timesheet Spreadsheet it shows that someone else is using or even that I already have the file open it, so it's just possible open as "read-only", but in fact no one really have the timesheet open it nor even myself. Basically we need to "save as" different name everytime this happens.

1) Why this is happenning?
2) How can I fix this problem?

Thanks in advance,


Jassiara Emmins