Sink rate is a function of pellet density relative to water density. Water density varies depending on TDS (total dissolved solids) and temperature. Pellet density is in turn a function of the material used to form the pellet (the food type) and the amount of air in the mix when it is extruded. Slow sinking pellets have a density that is close to equal that of water.
In the case of "sinking" pellets, the density of the pellet is much higher than that of the water, so it readily sinks. "Floating" pellets are the opposite, but usually will sink eventually as the air is forced out of the pellet by intruding water, increasing the pellet density over time. This is also true of "slow-sinking" pellets; eventually any air in the pellet will be displaced by water, increasing the density of the pellet and increasing its sink rate. You might be able to speed up this process by cutting a slow-sinking pellet in half. You can also speed up your sink-rate with a given brand/batch of slow-sinking pellet by decreasing the temperature of your tank or by increasing TDS (for example, adding salt).
