There are four possibilities. From worst to easiest to fix: (don't worry, a link provided below has diagrams of all the parts in exploded view with part names)
1. There are two springs holding the brake shoes firmly against pivot post and brake lever cam, the ends of these can wear through and break. These provide most of the spring tension felt in a cable operated expanding shoe brake. Requires wheel removal and simple removal of brake hub to access.
2. The cam that expands the shoes (when rear lever is actuated) runs through a waterproofing rubber grommet. This grommet can often tear and bind up, reducing the return spring's effectiveness.
3. There is a spring behind the brake's foot pedal frame mount and the plate that pulls the front of the cable end. This maybe either dislodged or the angled ends broken off.
There is a compression spring at rear wheel end of cable, between cable stop and lever arm. This can be removed, stretched and replaced or packed with spacers to provide additional spring.
Check site below: Wheels/tyres/drive - Rear wheel parts #6 (shoe springs) part #7 (brake cam)
Frame/Footrests - part #4 (pedal return spring) part #9 (compression spring)
If this helps let me know below, ...Cheers.