Some Amazon sellers (like you) find themselves unable to get 'buy box' exposure when they are among other merchants (including Amazon) selling the same exact product.
(Sellers 'win' the buy box when a buyer selects "Add to Cart" on the main product page AND the item displayed says, under 'In Stock', "Sold by [Amazon Seller's name]"). See an example here.
Amazon/eBay megaseller Skip McGrath just shared his strategy for getting the buy box, in his latest newsletter
(You don’t have to buy anything from him to learn from him.)
Get his free tips by going here and then then selecting “Newsletters -> Current Newsletter” – The first linked post at the top of that page is:
1. How The Buy Box on Amazon Works and How You Can Win It
also check out (the same page)
4. Setting up and Organizing Your eBay or Amazon Business