If you are enthusiastic about a subject, it can sometimes blind you to what the question asks for.

The mistake you make is to write about what you know, rather than what you have been asked to discuss.

You might not be sure where the emphasis should lie.

Example

QUESTION: "Discuss the influence of Thomas Hardy in the work of D H Lawrence".

Someone who was keen on Thomas Hardy might easily produce an essay which described the strengths of his writing, giving examples from his novels.

But the question asks you to discuss D.H. Lawrence - but with Hardy's possible influence in mind.

In other words, the emphasis in this question is on Lawrence, not Hardy.

Solution - as ever

Read the question very carefully.

Identify the key terms and instruction terms.

Ask yourself "What is the main issue?"