either your shoes are worn or not adjusting properly..
either case you have to take the back tires and drums off
check and see how worn the shoes are
if worn out replace them
otherwise the self adjuster is the gear shaped thing on the bottom that spreads the shoes out. spin it to spread the shoes out farther so that the drum fits on snuggly when you put it back on...
you need new pads probably. My Ebrake wouldnt work regardless of how adjusted they were with my old pads (worn to rivets) after replaceing pads and drums they work Very well, (almost too well)