If you look in the mirror without a top, and look at how the bra looks on you-does it look like it is digging into your back? If yes, try going up a back size. Are the cups fitting correctly? You shouldn't be spilling out or equally having a gap. Do go up or down a cup size if one of those is an issue. If all that is okay, it might just be the design of the bra that isn't working for you, rather than it being the wrong size.