There can be a number of legitimate excuses for bad grades. One could simply be that the student does not perform well in test situations. Another could be that the student is struggling with a learning disability such as dyslexia or a condition like attention deficit disorder.
Some students have bad grades because they are not interested in the material or are bored by the material. Others simply do not pay attention in class, so they do not have the information needed to prepare for tests. Circumstances like an illness or death in the family can explain temporary bad grades.