Need some more information. Is the container field by any chance using Global storage? If so then it is limited to that user and goes away when the user disconnects from the database (unless they are the host computer using FileMaker Pro instead of FileMaker Server).
Are all the users able to access the other data in that record just not the container field?
Another possiblity is the way the document is saved into the container field. When selecting the file/document/image, there is a checkbox in the dialog box labelled "Store only a reference to the file". If that checkbox is selected, the file will not be available to other users unless the path is the same for everyone (for example, on a network drive with the same path for everyone). If the original document is on that users local drive, then other users will not be able to get access to it in the FileMaker database.
Try loading a document with the "Store only a reference to the file" de-selected and see how you go.
I bet this is the answer. I've send along your suggestion to the user. Thanks so much!