Buybox confusion

Hey guys so recently i got a percentage of the buy box on one of my products and i started getting some sales. Today i wake up to zero sales and realized ed the buy got taken away from me. My question is how does it choose which offer to pick? My offer was a dollar lower than the new buy box. After that they gave another offer the buy box of which i was 2 dollars under and im fba with prime shipping? Is there something that im missing? And what does buy box percentages actually mean?