If you know that students are having trouble with a particular topic, I would offer to host an extra information session, but get them to bring along their questions and specify which parts of the topic they are having trouble with. That way you can understand the exact issues and perhaps put a plan in place to address them, rather than having to cover the whole topic which may be quite broad. You could also see if some of the students that are not struggling would like to buddy up with the ones who are struggling and offer their support.