Very often, when a student submits mediocre work, it is because they have left out the details and examples that prove their point.
If I asked what you liked about the beach and you just said “sand,” I would walk away a bit bored and wanting to know more. The same is true of your writing. If the prompt poses a question, answer it and then explain the “why”. Give details and examples and use descriptive language whenever you can.
“What do you like about the beach?”
“I like the sand best because I have a shell collection, and whenever we go to the shore, I look for colorful additions to my set. I also enjoy building sand castles in the wet sand with my little sister.”
Now this is a full reason with detail and explanation. It also uses adjectives and descriptive language to let the reader form a picture and impression of his own.
Always go back and infuse your work with details and examples that bring your ideas to life!