What's low? It depends on what you want. For example, if the grades are Ds and Fs, that is low. But also, if you want an A, then a B is low for you.
It can get worse with time
If you fall behind one year, how will you do next year? Worse? Probably, Here's why.
The reason is because each grade becomes harder. You have to know things from the year before. You cannot learn multiplication until you have learned addition. It just will not make sense, neither will division, or fractions, or anything else that is more advanced than addition.
In language, it is also true. You cannot read new words, if you do not know the phonics rules. You cannot understand meanings of new words, if you do not know the meanings of the root words.
Gaps are the main reason children struggle in school
When a student is missing knowledge from before, this is called a "knowledge gap."
Let's say, you do not know how to fix a jet engine. Now imagine you are a student in an advanced jet mechanics class. What would you do if the teacher asked you a question? Or asked you to show the class something on the board? Or gave you a test with 30 problems to figure out on your own?
It's not fair because you do not know the material from the classes from before.
If you tried to pass the class, you would really struggle. If you did not try, you would give up. Either way, it would be hard for you. This is what it is like for a child that is missing knowledge from before.
Motivation
Sometimes you cannot tell if the student is unmotivated, or just doesn't know the material. The reason is because he or she can do some things, but not others. This is exactly what happens with gaps.
Having knowledge gaps does not mean you don't know anything. It means you know some things, but it also means you are missing important concepts that you should know by now.
Students feel good when they do a good job because they know what to do. Children are people, just like everyone else. We all feel good when we can do a good job, and then we do it.
At the same time, we feel bad when we can't do something. And if that happens enough, we will no longer be motivated.
If you cannot tell if your child is lacking motivation, or just cannot do it, then it is probably both. They don't want to try because they know, they probably cannot do it.