Amazon's purchasing decisions are supported by an algorithm that is proprietary to Amazon. In choosing to stock titles or even show products as available on their sites, Amazon considers many pieces of information about your book. Factors in a decision to stock a book may include discount (with a maximum trade discount being most favorable), whether a book is returnable, product page views, and sales history. Even print on demand books may not show an 'in stock' message at Amazon. Amazon may also list your title as available through third-party vendors.

Although we ask them to maintain an "Available" for purchase status on our listings, it's not something we can require of them. If there is an "Add to Cart" button present, they are receiving, placing, and fulfilling orders through us in the normal POD (Print on Demand) fashion, they just likely don't have any units available in the warehouse. 

If a title has been on the market for a couple of years, active marketing or creating frequent social media posts will generate more traffic to the title on Amazon's website and will create more orders to come in.