Ha! My guy is exactly the opposite- will only eat food while it floats, which limits what I can give him. I wish I had your 'problem' then he could eat any time (everything sinks eventually) and he'd be helping with cleaning his bowl too.
I have one girl like that as well; as has been said, so long as she is eating then it doesn't matter. After a few weeks she did start coming to the surface for food.
I like feral also have a male who is the opposite; if it's more than a cm below the water surface he'll just ignore it. Where all my girls will pick the stuff from the bottom. Although if I feed him flakes I need to point out each individual one otherwise he won't eat it (needless to say he doesn't get flakes very often).