If a digital copy of the dissertation you want is not available, use GW Libraries' Interlibrary Loan Service to borrow a paper version or purchase a personal copy directly from Proquest/UMI
In some instances, the author of a dissertation may place it under "embargo". In that case, even though it's been "published", copies are not available. This is the author's decision and is respected by the databases and by the author's home library.
In these cases, GW Libraries will not be able to obtain a copy for you.
More on embargoes is here.