Crappies also tend to like dirtier water than a lot of other fish. Don't be afraid to fish shallow below your float as the other guys are saying and keep changing your depth until you connect. Slip floats are great for this. Some good systems for crappie are the Grand, Welland, and Martindale systems, as well as Beaverdams/marlatts.
In spots with clearer water, nighttime can also be good as crappies have excellent eyesight.