This is a very frustrating issue we are seeing with our students. Students get access to the prelab one week in advance of the lab. In the prelab, the students often have to use theory to predict a result in the lab. Almost all students get the prelab correct, and they know that they are correct because it is an online quiz type thing, so they get immediate feedback*.
When they get to the lab, they are asked a question like "what is your expected voltage, and how does it compare to the measured voltage?" A very large number of students, I'd say around 40%, will invariably ask "how do I know what my expected voltage is?" However, this would come from the stuff they did in the prelab.
So, how do I get the students to recognize that the prelab isn't just an assignment, but is actually a vital part of the lab?
*OK, so realistically it's possible that the students are all just taking one person's answers and therefore aren't really thinking about it, but my experience with this class is that would be the exception, not the rule.