For a document to be opened offline, a "Principal Key" for the user who has "offline" permissions is required on the client system that is being used to view the document.
If the document has been opened while online, then the synchronization should be occurring in the background. If the document has never been opened, and no synchronization (of principal keys) has occurred, then the document will not open offline. Each time a connection is established between Acrobat or Reader with the Rights Management server, the client should synchronize with the server and get any principal keys that related to documents that the user has offline permissions.
Try forcing a synchronization, from Acrobat select Advanced > Security > Adobe LiveCycle Rights Management > Synchronize for Offline. If you are using Reader, select Document > Security > Adobe LiveCycle Rights Management > Synchronize for Offline