Not all pond fish will take food from the surface of your pond. Sterlets, Carp, Tench and Rudd are just a few of the fish species that we class as bottom feeding fish due to the fact that they prefer to eat food from the depths of the pond that they inhabit.
Sinking fish food is specifically designed to accommodate these fish and therefore generally dense in its formulation so that it does not float on the water's surface. It is important to keep an eye on how much sinking food you are feeding your fish as it is easy to overfeed this specific type food due to the fact that it can be difficult to judge if it is being consumed or simply rotting on the base of the pond.