Audiovisual materials may be more difficult to obtain than print materials and, therefore, may take longer to arrive, if available. I requested something in plenty of time, with all the correct information. I was even willing to pay for it. Interlibrary Loan is able to fulfill the great majority of request, but some items are impossible to obtain. This is particularly true for very old or very rare items or dissertations.
Many institutions are reluctant to loan items of which they own the only copies.
Also most institutions will not lend non-circulating items such as reference materials.