Excel deleting sheets in shared workbook

[Mile029]

We have a shared workbook that about 15 staff share and update info on their assigned sheets. Lately when we open the workbook some of the sheets are missing, they are deleted and no longer exist. This is a problem since these sheets link to other workbooks and when they disappear they cause errors in the other workbook. Any ideas why this is happening? Thanks.

[Mile029]

I guess no one is having this problem? Any ideas would be appriciated. thanks.