Some instructors choose not to place materials on reserve because they provide electronic readings or prefer that their students purchase the text. Others may intend to put items on reserve but have to delay while they finalize their syllabus. Some reserve items may be on order for the library and simply have not yet arrived.
If your instructor has indicated that the readings are on reserve and they are not, please let your TA or your instructor know as soon as possible. Because the library relies on instructors to provide the necessary information on which books they have chosen to place on reserve, alerting the library will not make the reserves immediately available, but may start dialogue between the library and the instructor. A course reserve guide for instructors and their TAs is available here.