wessex lift boy problem closing?

my door will open everytime but when i want to close it, it looks like the small black bogey disengages itself and pops off the worm drive leaving the door open...if push the bogey onto the worm drive and hold it until the door begins to move then the door will close the rest of the way on its own...is there an adjustment or is it a replacement bogey? Many thanks

mikeyboy
April 2012