I am facing the exact same issue and got a very long unsatisfactory answer from amazon as follows
Greetings from Amazon Seller Support,
We appreciate your patience while we were working on this issue.
From your correspondence, I understand your concern regarding winning the buy box eligibility for listings in your account. I will do my best to assist you in the best possible way.
–> Hello, can you check if there is any issue with amazon buy box, our buy box is down more than 50% in last day
This percentage indicates how frequently your offer is featured when a customer views the detail page. Sellers who win the Buy Box have a greater chance of gaining the sale.
Based on your comments, we reached out to the Buy-box eligibility team and they have confirmed that the buy-box eligibility is working as intentional.
They have also checked a few of your listings and confirmed that there are no issues with your listings and our system is working as expected.
I understand that your products are eligible, however, since the buy-box eligibility is calculated on a periodic basis and is completely automated, we cannot manually adjust it.
Also, as mentioned above, as a onetime exception, we escalated this to the technical team and it has been assured by them that the buy-box eligibility is working as intentional, and a manual adjustment cannot be entertained as it would not be fair to other sellers.
For business reasons, the logic by which the scores are calculated for buy box winning offers, the cumulative score of a seller and other sellers is kept confidential. Even Seller Support associates does not have permissions to view this data. I’ve already contacted the concerned team and they confirmed that the other sellers offer has a better score than yours which is reason for winning the buy box.
Pleased note that at times Buy box is won by Amazon or by the sellers simultaneously. Also, I’d like to inform that, there are instances, where the Buy box is won only by third party merchants, even if Amazon is listing against the same offer.
Sometimes the buy box winner, holds the details page for just a minute and the winner may change and sometimes the same seller may continue to win the same buy box continuously. Also, that since the concept being an automated process the eligibility criteria is met and still not won.
For example, sometimes it happens that comparatively orders are received to a particular seller’s or Amazon offer and so the buy box is automatically selected. However despite good sales history as well the buy box is not won. As the criteria is of many other factors.
Please know that choosing a buy box winner is an automated process where a designed program selects sellers as per the metrics evaluation process.
Also know that numbers, parameters and metrics keep changing and there are scenarios where Amazon is offering a product at a cheapest price, providing excellent customer service and maintain a good feedback rating and still not winning the buy box being a lone seller of that product. Same is the case with thousands of sellers on Amazon.
There are cases, where, a product has more than 20 sellers offering it, however, not a single seller would be winning the buy box (same is the case with your listings as well). As you used to win the buy box before and not winning it all of a sudden is worrying you and we understand that completely.
As this is an automated process, we cannot guarantee what the system does while choosing the winner. Yes, there is a certain algorithm behind it like I mentioned, however, it is not known to seller support.
Buy Box eligibility is regularly checked and based on factors that have been carefully chosen in an effort to give customers the best shopping experience. There are several other factors apart from lowest price, that also influence Buy Box eligibility including: category, availability, products visibility, sales history and the seller’s performance.
Please kindly note that even if there is only one offer for a product, Buy Box eligibility is still evaluated and items must be priced competitively in order to be eligible to win the Buy Box. Offers that do not meet the criteria to win the Buy Box are still displayed on the Offer Listing Page.
Amazon does not guarantee that any particular seller, including those who are Featured Merchants, will win the Buy Box for a product. However, you can take steps to increase your chances of winning by following the guidelines in these Help pages:
The Buy Box is intended to showcase Pro Merchants with high sales volumes who consistently provide high-quality products, timely order fulfillment, and excellent customer service.
Please note that Seller Support cannot influence Featured Merchant status. If a seller meets the current eligibility requirements for Featured Merchant status, Amazon will automatically enable Featured Merchant status for that seller or the category team will contact the seller directly.