There are a few reasons why you may end up with a list of articles that are not what you're looking for.
1. If you get a list of very few articles, the selected keywords might be too specific.
In this case, you will need to be more general.
Instead of: body mass index of children in elementary school who eat doritos
Try: body mass index AND children AND junk food
2. If you get a list of too many articles that cover too many aspects of the topic, the selected keywords might be too general.
In this case, you will need to be more specific.
Instead of: plants
Try: invasive plants
Another way to be more specific is to put phrases in quotation marks:
"invasive plants" "soft drinks" "body mass index" "junk food"
3. Article authors may have used slightly different words.
For example, instead of soda, they may have used soft drinks.