LibAnswers

Although there can be many reasons for delay these are some of the most common:

• Lending libraries have their own workflows and timelines for responses. Some larger libraries (like OSU-Stillwater) have their materials spread out on several different locations on campus and items must be located and delivered to the main library and then ship it to us from there a location.
• Availability may be an issue. Although we can see which libraries have purchased a certain book or journal we can't see if the book is sitting on the shelf, lost, or checked out. Books in popular demand may be off the shelf at several libraries before we find a library that has one to lend.  Textbooks may be held for "in-house use only" at a majority of libraries.
• Shipping carrier and times vary depending on the location of the item.  Weather and natural disasters can  delay a factor in some delays and overseas book shipments take longer.  
• Sometimes, if the citation is incorrect or incomplete, the library that receives our request may take time to investigate to try and fulfill the item. 
• Rarely emergency situations or natural disasters can affect delivery of interlibrary loan items. Delays in mail, courier and library closures can affect response time.