yeah, they'll scrounge the bottom for food, and if they are hungry enough they'll go after some flake for themself.
if you dont' think its getting enough to eat though, you might try some sinking pellets. i think there are more than a few brands that make pellets for bottom feeders. (i'm picturing one with a cory on it... wardleys or tetramin, i'd guess). rainbow sharks are actually closer to a catfish or loach than a true shark.
you might try algae wafers too... theres always fresh or blanched veggies. many fish will eat these once their accustomed to the presentation. same goes for a lot of foods, actually.
if the top feeders are nabbing all the flake, you might try pinching some below the waterline... makes it sink right away.
my rainbow shark is super picky about eating as he has gotten older. I know hes eating because hes growing, active, and beautiful. Im sure he searches the bottom all night but his absolute favorite is frozen brine shrimp thawed in tank water. It doesnt matter what time of day....he will come and put as much in his mouth as he can fit.
Someone already mentioned veggies, mine just love the veggies I have put in there, I've blanched carrots, zuccinini, and celery so far. I weight them down with my plant anchors and my shark munches on them till they are gone...he chases anyone who even comes near away (MINE, MINE, MINE!).