How do I figure out what a struggling student actually doesn't understand?

A grade tells you a student got it wrong. It does not tell you why, and the why is where the real teaching lives. Feeling stumped here is not a gap in your caring. It is that the mistake on the page is only the surface, and the misunderstanding underneath is hidden. To find it, you have to stop looking at the answer and start listening to the thinking. A wrong answer almost always comes from a rule the student is applying that made sense to them somewhere. The child is not random. They are being logical from a wrong starting point, and your job is to find that starting point. Sit with one struggling student and ask them to walk you through how they got their answer, out loud, without correcting them. Just listen for where the logic bends. That single conversation will usually show you the misunderstanding faster than a stack of graded papers ever could. Teach the bend, not the answer.