Every boob has it's own personality. Some are perky, some are... the opposite. Some are fast learners, some are slow to catch up. Some are perfectly happy to stay smaller than their bunkmate. It's just how boobs are. They like do what they want no matter what their keepers desire is. Your little late bloomer may be taking the scenic route and will eventually get to the same place as the other. On the other hand, it might just stay behind. It might rocket out in front, and the other might stay the same size.
Just trust me on this. I see a good deal of ta-ta's in my profession, and most chicks are at least slightly lopsided. Some even need extra padding in their costume bra's to even things out. Course some want that extra padding just for fun.