Deterioration of neutron yield in a low-energy plasma focus operated by a single 32 mu F, 15 V, (3.6 kJ) capacitor is observed. When the cumulative discharge energy over successive shots across an insulator sleeve approaches 1.6 MJ, the neutron yield from the device starts deteriorating. The insulator sleeve, when examined, is found to have a approximately 3 mu m thick layer of copper deposit. The contaminated sleeve surface appears rough with a grain-type structure. It is therefore concluded that the degradation of neutron yield in the low-energy device occurs due to Cu deposit on the insulator sleeve surface. The situation may improve if low-sputtering-rate conductors are employed for the electrodes of the device.