The person struggling with instructions may understand the idea better than their work can show.

Following directions and understanding a subject are different abilities that get graded as one. Someone can grasp the concept completely and still lose marks because the format confused them, the sequence was unclear, or reading a long block of text is genuinely hard for them. What the finished work records is the combination, and the person reading it usually attributes the whole result to comprehension.

When work comes back weak, ask the person to explain the idea out loud before concluding anything. If they can, the problem is the instructions. Fix those rather than reteaching what they already know.