When I started having this problem, it was because the tiny little microswitch at the bottom of the power unit at one end was not being adequately depressed by the collector basket. I added some tape to the little ridge on the inside of the basket that depresses this microswitch and now it works.
Apparently this switch is for safety to inhibit operation if the power unit is not on top of the basket ready to go. In any case, good luck with your problem.
Dick
January 2009