From what you present it appears you're doing things in the way Amazon wants but as always there are more hoops to jump through. Just remember Amazon is putting its own reputation on the line with every featured offer it presents. Here's a few ideas that may help you.

1) Have you got a good history of shipping on time? If you're a new seller you may not, daft as it sounds even working FBA you still won't have a history of fulfilling orders on time. You need to build history via sales of your product or possibly another product (correct invoices etc) to qualify for the buy box.

2) Can Amazon ascertain that your product offers good value? i.e. is it offered via Ebay/your own site at the same/higher value. If not the fallback comparison will be items within the sector which may/may not offer value or even be direct comparisons for a multitude of reasons. Only items that offer value (in Amazon's eyes) will get the featured offer.

Put yourself in Amazon's shoes and ask why should they risk their reputation on an unknown seller with an unknown product? If you can create a reputation for speedy delivery and a demand for your product (via social media or other means) Amazon will be only too happy to give you the featured offer and take their referral and FBA fees.