Usually this kind of problem has nothing to do with QlikView, as QV isn't reponsible for authentication. Other systems take care of that side of the permissions game.
There is an easy way to check whether QV recognizes this user at all and what permissions he/she gets on specific documents. Open the QMC->Users tab and enter the NTNAME of this user in the search box. If nothing appears in the list below the search box, your AD has a problem reporting user information, OR your DSC is missing for this domain or not properly configured.
BTW is this user member of the same domain as the others, or is he/she member of another (trusted) domain?