Some themes are popular. They have been written about and examined by multiple authors. Sometimes an author returns to a theme multiple times, exploring it through different stories. And in some works, an author may weave multiple plots (some are sub-plots) within a single work. They can each reinforce the primary theme or raise questions about it. The best thing the author can do is not to put many simultaneous subplots. Many secondary plots can often confuse the readers, and they will not understand the main plot.