There's a good chance you could improve your teaching if you were to:
- relating the brightness of lamps to what's happening to the current and voltage in each loop
- using semi-quantitative reasoning and physical models, such as the rope loop
Work through the Physics Narrative to find these lines of thinking worked out and then look in the Teaching Approaches for some examples of activities.
- diving straight into the sums to be done
- using ad-hoc rules to generate the sums to be done
These difficulties are distilled from: the research findings; the practice of well-connected teachers with expertise; issues intrinsic to representing the physics well.