When floater fishing for carp, always ensure that the last foot or so of your leader sinks.
This avoids what fly fishermen call F.N.S (Floating Nylon Syndrome). This is where the leader casts a shadow, causing the fish to turn away from the bait at the last moment. Sound familiar?
Best way to do this is rub the leader with a paste made from Fullers Earth (available at your local chemist) and washing up liquid. This degreases it and has the additional bonus of removing some of the shine.