The so called BuyBox protection is enabled for top selling products (product sold frequently in the last hour) to prevent out of stock sales of popular products. Buybox protection activates if the offer has only few items in stock.
If your offer has only one item in stock, your offer won't be shown to the customer as long as the product remains in top-seller status. If your offer has two pieces in stock, only one piece is shown to be left on the website.