I don't have a lot of faith in cranks for crappie, but I have caught them on the smaller sized hard jerkbaits, when fishing for bass. Hard to believe you can't get bit on small soft plastics though. Try Bobbie's straight tail plastics on a 1/16 oz. jig head. I've never seen them fail when crappie are on a minnow bite.
Edit: You might also try a 1/32 oz. marabou jig under a bobber.