Dental filling materials (composite, amalgam, ceramic etc) do not decay but the tooth surrounding the filling can still decay even after the filling has been completed.
Plaque accumulation can roughen composite and damage the bond at the edges of the filling. Over a long time in the mouth, composite wear out and degrades, especially in areas with heavy biting pressure.
Ultimately, fillings ARE NOT A CURE for decay. Fillings are done as a last resort when the decay process has caused irreversible cavitation (formation of a hole/cavity) in the tooth surface.
Tooth decay is a disease of the mouth, not of the teeth, so proper prevention is required to change the oral environment to a healthier state. If this is neglected, having fillings done repeatedly will be a pointless exercise since at the end, the teeth will still end up becoming so destroyed by repeated decay that they are no longer structurally sound.
