Shared workbook losing filter function

[DBR]

I have an Excel workbook that uses a macro to generate approximately twenty different worksheets. Each worksheet has hidden columns (with a heading on each column); the data is sorted, frozen at a particular row & column and is also filtered. The page settings are also changed to legal, landscape, fit to one page wide and half inch margins. Each worksheet is protected and then the workbook protected and is shared, allowing filtering. The workbook can be modified by as many as twenty different people at one time, which is why the file is shared. After a short time however, users are experiencing an issue with the filtering in that it sometimes gets frozen, that is the drop down arrow appears but is not functional. If the file is then closed and reopened the drop down arrow is occasionally gone completely and because the file is protected the user is unable to add the filter back. I have also found when asking people to log out of the file so it can be fixed, they tell me they are not in the file even if the sharing window shows their user name as well as seeing duplicate user names. This is a monthly report and has had the same issue for the last several months. I have done some testing saving from different versions, 2003/2007, but that had no affect and I have not been able to recreate the issue. I am not able to find any information doing a search in Google or this forum. Any help in resolving this issue will be greatly appreciated.