If you are sure it is because the topic is too broad, then there are a few fixes.
1. Divide the topic into smaller, more concise sub-topics (instead of weather as a topic go for lightning, wind, or specific cloud types and how they help determine the weather for the day).
2. Relate the topic or subtopics to something else (i.e. glass = the science behind glassmaking, the art of glassmaking, recycling glass, reclaiming broken glass, types of glass service ware, the history of glass dinnerware).
3. Relate the topic to something local to you, like your state, region, or city (e.g. instead of geography why not try the Geography of Texas or the Geography of Houston and the East Coast of Texas. Instead of RIver Systems, try River Systems: the Brazos RIver in Eastern Texas.)