@LornaLove Here’s the thing, the most important measure is the back size, not the cup size. In America most of the better brands for large breasted women you measure in inches where the bra band goes beneath your breast and straight around your back and add 4. So, if you measure 30, you are a 34. Some brands you are supposed to add 5. Either way, most people wear bras a whole size bigger if they are in the wrong size.
People are wearing 36 when they should be in a 34. Once in a while I came across some in a bra that was two small around the back, but rarely. The bra should be flat against your chest, doesn’t move when you lift your arms. When you have that size right, when the cup is right the wire is against your cheast wall, not up against your breast tissue, and your breasts are well inside the cup, nothing spilling out the front. If you have the wrong size around the back you can still fit the cup, and that is where people tend to screw up. For instance if you need a 34DDD, if you put on a 36DD, the cup will fit well, but the bra will slip around. None of this might be the problem in your case, but we found this problem a lot with women.