One of our users just brought this problem to our attention, it seems like Amazon is telling SE Plus you have the buy box when in fact you no longer have it for items you indeed had the buy box previously.
One scenario in which this happened repeatedly is when an FBA seller maintains the Buy Box and the user's listing is merchant-fulfilled, the software indicates the user has the buy box.
There's a discussion about this also on the Amazon forums, you can find it here: https://sellercentral.amazon.com/forums/thread.jspa?threadID=166021&tstart=0
If you are having similar issues please add your experience to the thread to help us narrow down the problem scope and report this to Amazon. Thank you for your help!
Seller Engine HelpTeam