Since COVID hit, we have been seeing a slight change in the buy box. It tends to rotate a bit slower on some listings, and this is most likely related to seller inventory volume (sellers with more inventory) and seller location. If a fulfillment center is full and Amazon needs to make room due to the increased demand of online shopping, they may prioritize the sellers who are at that location. So, there's a chance something like that is happening and is making it a bit difficult to get the buy box.
As long as you are selling New, selling FBA, and matching the price, it should end up rotating to you. Keep in mind the buy box does rotate evenly, however it’s over the course of a week, a couple weeks, or a month. We have seen this happen with another student where they did not get the buy box for some time, but when they finally did get it, they ended up holding it for quite a while, so it did end up evening out.
You could also get in touch with seller central and see if they can spot any issues on their end.
Why is the buy box not rotating evenly?
How does the buy box rotate?