So I got my binder and even though it flattens my chest a buttload, I have one problem. When I adjust myself so that it flattens my chest better, my boobs go back to the exact same spot within a few minutes. I was thinking that maybe I didn't get a tight enough binder, but I think if I was to go down to the next size I won't be able to breathe properly - and I quite like breathing.
Yes try on another size, I think you have to treat this as buying any other item of clothing; Your not always going to get a perfect fit first time. I have a very large chest and bra shopping is challenging for me because it takes a long time to find the right fit and I have to try on many different bras and I find (as you'll most likely find with binders) that different manufacturers products fit in different ways so switching to a different brand could also work for you.
You could also try wearing some kind of sports bra underneath it that might hold them where you want them.
Before you go down to a smaller size, a few things: 1) Contrary to popular belief, you should be able to breathe in your binder. It may not be particularly comfortable, but you should be able to breathe. 2) How are you placing your chest, for lack of better words? A lot of people, including myself, say they find it best to place them to the sides (towards the armpits) and push down. 3) Instead of a different size, maybe try a different brand as Toffee said, or even a different style. There are full-length ones that cover your entire stomach, waist length ones, sports-bra length ones, t-shirt style, tank top style, v-neck style, ones that are made of just spandex, velcro ones, etc. When I was heavier, I found that the full-length binder wasn't that effective, but now that I lost a bunch of weight, it is. So, different styles work better for different body types and people. What works for someone else might not work for you. 4) To a degree, you might need to learn how to move again with your binder. If you notice certain positions (that you can avoid or alter) are causing your problem, see if you can do something about that. If you're a very active person, this is probably not a valid option though.
