If you put spectrum pellets in a small cup or shot glass of tank water and give it a stir, they'll sink. Then just dump them in the tank and they'll go right down. The problem with pellets is that they have little air pockets in them that make them not as negatively buoyant as we would like.
I have tried many pellets for picky fish and the one pellet they love is PE Pellets 1mm They sink right away but you have to place under the surface of the water to break the surface tension. Everything in my tank loves this pellet including coral.