librarygirl-as you lose weight, you may find that you have less trouble with the shoulder straps on the bras...
When bras are made larger in the band, the whole frame of the bra is larger. If you have a 32 band bra, and a 40 band bra laid out together, the straps are further apart on the larger bra.
If you are naturally smaller framed with narrower shoulders, you may experience strap slipping, even if the ribcage band fits. As you lose weight and go down to a smaller band size...your ribcage (because of less fat covering it) will be more proportionate to your skeletal frame. Your straps will be closer together, and the fit will be better.
Patience!