Like any other subject, science builds upon itself chapter by chapter. Introductions to physics, chemistry, and biology are designed to set foundations for harder scientific concepts, but in themselves are mostly memorization for the first semester.
The second semester of physics, chemistry, biology, and other sciences like environmental science are much more comprehension based. The pure factual information retained is required to make larger leaps in science, and in that change from memorization to application, many students fall behind. For introductory classes, I make sure students understand the foundations they need in order to help them with more complex topics.
For upper division classes and AP courses, science can just be plain hard. There is nothing wrong with struggling in the sciences, be it because of differing personal strengths, a poor teacher, or stress from other classes. My associates and I are here to make sure the learning process isn't stressful or overwhelming. Correcting just one misunderstanding can prevent hours of frustration.