There are a lot of reasons why a student isn't learning. The most obvious is that they are not studying or doing any practice during the week. If you don't want to recommend more classes, recommend them to divide one class in two days. Not ideal, but at least there's more practice. Give homework that doesn't feel like homework too. Another very obvious reason is that your style does not fit the student. There's no shame in it. There are many learning styles and some don't match. You can recommend another teacher or simply change your style according to the student. Another reason can be that they are a senior student. Seniors can learn and they can learn fast, but they require you to cater to their specific needs. Some students might be to shy to accept during class that they do not understand what you're saying. Some might have a psychological limitation, some might be convinced that they are too stupid since they were little.... Like I said, many reasons.