Some possible reasons for this could be:

1. Lack of motivation or interest in the subject: If a student is not interested in a particular subject, they may not put in the effort to understand or learn it.

2. Learning disabilities or difficulties: Some students may have learning disabilities or difficulties that make it harder for them to understand certain concepts or topics.

3. Lack of effective teaching methods: If the teaching methods used by the teacher are not effective for a particular student, they may struggle to understand the subject.

4. Distractions or other external factors: External factors such as distractions, personal problems, or lack of focus can also affect a student's ability to understand a subject.

5. Language barriers: For students who are learning in a second language, understanding complex subjects can be more challenging due to language barriers.

6. Lack of foundational knowledge: If a student does not have a strong foundation in the basics of a subject, they may struggle to understand more advanced concepts.

7. Limited resources or access to resources: Some students may not have access to the necessary resources, such as textbooks or online materials, to fully understand a subject.

8. Different learning styles: Students have different learning styles, and if the teaching methods used do not align with a student's preferred learning style, they may struggle to understand the subject.

9. Lack of support or guidance: If a student does not receive sufficient support or guidance from their teachers or parents, they may struggle to understand a subject on their own.

10. Lack of confidence: Some students may lack confidence in their abilities, which can hinder their understanding of a subject.