Why does Powerbot VR7000 stop after a few seconds

Brand new Samsung VR7000 worked beautifully three times. After that, it operates normally for only a few seconds at a time and then stops. Indicator lights show that it is fully charged. I have tried turning on and off the power switch on the underside, I have cleaned the sensors, emptied and cleaned the dust container and filters… Am at a loss as to what I should try next. Any suggestions welcome.