If you are a private label seller and are not holding the Buy Box, check the following factors:
1. You have been selling on Amazon for less than 90 days. If not, all you can do is wait and make sure your seller performance metrics are on point
2. Your seller metrics are not on point. Amazon gives the Buy Box only to sellers who perform. They look at metrics such as Shipping Time, Order Defect Rate, Response Rate etc.
3. You increased your price too much. (See price limits for minimum eligibility to win the Buy Box)
4. You are not registered for VAT or there are other legal issues.