Professional bra fitters can tell immediately why a bra isn't working on you. Some of the clues are:
1) If your cup is wrinkling, you need to go down one bra (not cup) size.
2) If your breasts spill out the sides or over the tops of the cups, you may need a larger cup size or a cup with more coverage on the sides and top. For example the Fantasie Smoothing Molded Full Cup Bra (http://www.inthemoodintimates.com/intimate-apparel/4500.html)
3) If your straps keep falling down, even after adjustments, your breasts many not be filling out the top of the cups. Try going down a cup size.
4) If the bra rides up in back, try a tighter hook or loosen the straps. If neither works, go down a bra size.
5) If straps leave marks, it doesn't necessary mean they're too tight. You may need a more supportive bra or you may need shoulder cushions (http://www.inthemoodintimates.com/intimate-apparel/6100.html)
6) Your bra straps are at the correct length if you bra is level across the back and not too tight when using the middle hooks. The middle of your bra should lie flat against the chest. Cups should be full with no spillage.