In most well-structured paragraphs, a topic sentence is followed by supporting details that explain, clarify and prove the statement in the first sentence. According to Laurie Miller of George Mason University, American writing typically starts with a broad topic statement followed by more narrow supporting details. If your instructor says your paragraph is not fully developed, she may mean that you do not have enough supporting details to fully explain or prove your topic sentence. For instance, if you write a two-sentence paragraph that asserts students should not be required to take non-major classes because it harms their GPA, the paragraph is poorly developed. You have failed to include why non-major classes would harm students' GPAs and why your audience should be persuaded by the GPA evidence to reconsider non-major classes. (See Reference 3.)
A specific kind of supporting detail that you can use to develop your paragraphs is the example. Whether you choose hypothetical or real examples, this tool can help you fully explain or prove your paragraph's main idea by giving your readers something concrete to which they may relate. Sometimes you need examples to qualify your statements. For instance, if you claim in a self-evaluation essay that you improved your study skills in the course, you need to give a few real examples to show that this is true. Other times, hypothetical examples can make abstract concepts, like theories in art or social studies, easier to understand.
If your teacher has required or encouraged research in your paper, poorly developed paragraphs are a naturally occurring result of too little research. When you make claims in your writing, most teachers require that you back up your claims with facts, statistics and the opinions of experts. For example, if you write that students who do not eat breakfast score lower on tests than students who do, but you cite no scientific evidence to prove this, the paragraph in which you discuss this idea will be poorly developed. To develop these types of paragraphs, quote, paraphrase and summarize resources that support your opinion.
In creative writing or some compositions in which you are allowed more room for creativity, description can be the key to developing paragraphs. You may want to add details that describe the setting, characters or actions. Do not simply add descriptive details to increase your word count, but do so in paragraphs that fall flat because the reader cannot envision the setting, the characters or what is going on. You may also add description to help you use other literary devices like symbolism, foreshadowing and character development. For instance, if you want to foreshadow a character's death, you might develop a paragraph by adding description of the character's macabre thoughts. This helps your reader understand the purpose of the paragraph as well as the character about whom it is written.