Pellets or whole leaf? Pellets will usually sink, but leaf tend to float the whole time. Assuming you're using pellets, though, the particles cause nucleation sites for CO2 to come out of solution, and the pellet mush traps the bubbles and they end up floating. It should sink after a day or two, though.
If it worries you, I like to give the fermenter a quick twist, which helps knock them down to the bottom. Be VERY careful if it's in a glass carboy. And if you can cold crash, even as little as a 10°F drop will cause them to fall.