You most likely use 21st century digitally enabled book-marketing actions. However, your sales are probably occurring at dial-up speed if you are implementing 20th century marketing actions.
You wouldn’t put up with a 1960s-era phone system, or forego the benefits of modern technology, but that is what you are doing when you fail to sell your books to non-bookstore buyers. These could be retailers (discount stores, supermarkets, airport stores) or non-retail buyers (corporations, associations schools, military) who could buy in large, non-returnable quantities.
Selling to these special-sales buyers does not have to be a major leap. You already have the programs in place to do it. Selling to non-bookstore retailers uses many of the distribution partners with which you already work selling through bookstores. Begin there, and then expand your sales to build upon that base, giving you a solid foundation and focal point to grow your sales.