If there is no Buy Box displaying on a product, it is because Amazon does not think there are any good offers to show customers.
This could be because all sellers are priced too high compared to historical prices and other prices online (like Walmart.com and Target.com). It could also be that there are no FBA sellers and no quality FBM sellers.
Usually, if there are New-FBA sellers on the listing but no one is winning the buy box, the reason why there is no buy box is because those sellers are probably priced much higher than past buy box prices. You can tell by looking at Keepa.
In the example below, you can see on Keepa that the last time a buy box was displayed (Pink Diamonds on Keepa) the price was about $10.If you look at the offer page, you will see the lowest offer is currently $20.18 which is more than 2X the last buy box price.
