Perhaps after observing this several times you could summarize the key points that these students are missing and need to know in order to move forward with the current material. You could create and post a prerequisite knowlede page. That way students could read this at the beginning of the program and brush up on items they are fuzzy on. You could include some appropriate resources or links to good resource sources.
Alternatively or in addition to that you could speak to the admissions people at your institution about what level is really required to be successful.
Your question has made me think about how I can better accommodate different levels of learners in my own classroom!
