Order of lessons in some subjects

Hello I am absolutely loving what this website offers and it is proving hugely helpful to me. However now and then I will be working on a set of questions and I will feel like I have missed a fundamental part of the class because I will not understand something and it wasn't explained in the lessons. So ill struggle my way through the questions, then I get to the next set of lessons and suddenly it is videos explaining what I was struggling with in the last lessons questions. I especially notice this in the chemistry classes.