Roxanne, my instinct would be that you simply need a bra in which the cup fits you better than the one you have, even if you're technically sized correctly.

Not all bras are created equal, and neither are all breasts. I have a large cup size, but my breasts are not incredibly "deep" - a bra that has a lot of volume in the lower part of the cups is going to leave me with wrinkly cups and excess space at the top, even if the bra fits well. Through trial and error I've learned that a balconette style fits me best. My breasts do come out a bit far under my arms, so I need a bra in which the underwire/cup is wide - otherwise I end up with unsightly underarm overflow.

I'm not sure whether I am explaining this very well, but I find these troubleshooting threads very helpful:
http://www.figleaves.com/us/fi.....8;txt=5673
http://www.figleaves.com/us/fi.....8;txt=5683