I had a similar problem (cups fit, band SEEMED to fit, falling straps), thinking bras were just too short for me. Turned out I needed to go from a 34C to 32D--I was basically proportionately smaller, even though the cups still fit somehow. (Lost 10lbs, even though I only noticed it all of a sudden, too lol)

Check your current bras by hooking them on the last set of hooks and see if they still fit the same. You might discover that while the cup fits, the band really doesn't fit. Otherwise, you might just need a bra from a different brand. However, I think SM is right, especially since you lost quite a bit of weight--try going down a band size and up a cup size, trying different brands.