Hmmm, that suggests you need wider wires and would explain the issue - it's not that you are too small for the bra but that some of your breast isn't in the cups because the shape is wrong. Does that make sense? I'm guessing your breast tissue goes quite far round (i.e. the base of your boobs is quite wide) but you don't have much projection?
Unfortunately a lot of bra shopping does involve serious trial and error - some bras just never work. I have quite wide based boobs and Panache is my bitter enemy
but I get on well with quite a lot of Bravissimo branded bras and some Freyas (got on well with most of them when I was smaller titted!)
If you can, I would suggest ordering a variety of bras - unless you have somewhere good for fitting locally - from different brands and with different shapes. You also might want to order the 28D and DD. The wires on a DD will be wider and it sounds like you might need that. Bravissimo have recently started stocking a brand called Tutti Rouge which is supposed to have wired which are very wide, and quite shallow cups. Bravissimo own brand half cups might be an option, and I also find the wires on the Bravissimo Alana to be quite wide (and others too). Possibly also try some Freya bras.