All whole FD foods float until you remove the air pockets inside them. I soak whole FD critters for several minutes. Then, gently press them against the sides of the bowl while still submerged. This removes and replaces the air with water and the food will sink.
You should always soak freeze-dried food before feeding. If not, it can expand in a fish's stomach, and that's a major problem for greedy fish that like to stuff themselves. It can end up doing permanent damage or killing them.