Maybe I did this right.
In this picture the front of the truck would be on your right, so this is a passenger side set up(it will be a mirror image for the driver's side).
I found this on google so it's not an exact match but it will work.
You should have a lever that comes down from the secondary shoe and hang over the 'star' wheel of the adjuster. When you apply the brakes in reverse, the servo(rotating) action of the brake shoes cause the lever to come down, turning the star wheel. Thus, self adjusting the brake shoes as they wear.
Hopefully the picture shows up, and hopefully that helps you diagnose your problem.