Document changes aren't saving

Have a small network of XP Pro machines - 5 machines - and users are running
into a strange behavior. There's a NAS device (D-Link DNS-323) that has a
shared folder where all users store their common files and all users have
unrestricted rights to the folder.
Not infrequently a file will be opened for edits, e.g. last month's
newsletter to be changed to this month's, the document is edited and saved
under a new name, but when the new file is re-opened the edits are gone. The
files aren't being opened simultaneously by multiple users, it's strictly a
one at a time situation.
Any suggestions to resolve this?