I have 3 bala sharks, 2 of them eat flakes but one doesn't want anything to do with them. I have to crush up shrimp pellets and feed him that (he's big enough now to eat whole ones) but even dropping stuff to the bottum will allow them to eat when you're not there. They are very finicky and are often scared by your presence. If you stand there watching they probably won't eat until they have become comfortable and that usually takes weeks if not months. My sharks will swim to the opposite side of the tank when i enter the room unless its breakfast time haha then they stay as close to me as possible just to make sure I don't forget them. so try dropping sinking shrimp pellets in or even algea flakes. Also flake foods that are veggy based are great for them. you can even try very small goldfish floating pellets, they like to attack stuff on the surface and make clicking noises.