1. If you get very few articles, your keywords might be too specific.
Instead of: elementary school children who eat Doritos
Try: children AND junk food
2. If you get too many articles covering different aspects of the topic, your keywords might be too general.
Instead of: plants
Try: invasive plants
3. Article authors may have used slightly different words.
For example, instead of junk food, they may have used ultra-processed foods.