Possible reasons:
The notes database doesn't have a default view, so the link cannot be accessed.
The document is not readable by the user who is opening notes.
The database is not accessible by the user who is opening the link.
Notes took so long to open that Outlook gave up.
Notes is waiting for the user to enter their password.
Notes has a dialog box or prompt box open that prevents a document being opened.
The document is not in the default view. (this is a long shot, as it shouldn't matter)
The default view is a private view and the linked document is not available to that user's version of that private view. (Set the default view to be a hidden standard/shared view in this case).
The user has Release four of notes and already has 9 windows open.
Another reason I have found out about which is hard to nail down is that MS have changed the way that Outlook processes these objects and have removed the use of the REM or HINT tags which hint at which server to use. I have not worked out which OS / Outlook Version / Notes Version this affects so if you work it out let me know.
Solutions:
1) You can wait for MS to change Outlook so that it works with the world's leading groupware solution. Hint: Don't hold your breath.
2) You can convince your IT Director to go back to Notes mail.
3) You can add links to the database as well as to the document to your mails, and also add Server, Path and view and document title information, to help the user manually find the document.
4) Add a url link to the document instead of or as well. MS cannot stop them from working can they.
5) Send the document to a Developer Notes Email account and check the links work from there, it might be something silly.
6) Create .ndl files and send them instead of doclinks as attachments.