The most likely reason for this is easy to miss. It is because the assignment or expectation was not clear. Maybe it was not explained in a way your child could easily understand. This is usually the trouble if the problem comes up mostly with one teacher or with another adult. If the problem often comes up with several people, something else is likely going on.
There are several possibilities. Put vision and hearing problems at the top of your list. Even if the teacher tells the child about the assignment, vision problems still need checked. Gestures, facial expressions, and other visual clues are important parts of listening and understanding.
If the assignment was in writing, hearing is important. The teacher probably said something when giving the assignment. Other children may have made comments that could serve as clues. Any time your child has learning problems of any kind, have his hearing and vision tested.
Note whispering and talking quietly do not adequately test your child's hearing. It is the same idea as simply looking at objects or pictures does not adequately test his vision.
Maybe he did not understand because he was not paying attention. This could be because he did not think the assignment was interesting or important. Maybe he figured he would screw it up anyway. Children with low self-esteem often think there is little point in trying.
Two other possibilities need some thought. He might have special problems paying attention. For example, he might have Attention Deficit Hyperactivity Disorder (ADHD). Be sure your child is seen by a psychologist or your doctor to check this out if he consistently has trouble paying attention. Your child will be checked for other possible problems at the same time.
The other possibility is harder to check. The assignment or what the teacher expects may be beyond your child's learning and experience. He has not learned what he needs to know to understand the assignment. This may be because his education has been neglected. He may have serious mental or emotional problems. He just may not have the needed ability. You may not want to accept these as possibilities but your child deserves consideration of every possible explanation.
Your child's not understanding can be complicated. Keep an open mind. Encourage and support him while you check it out. The key is not to do anything he may read as blaming him. It is not his fault he does not understand. There likely is more to it than his simply being difficult and not cooperating.