Kami extension only working for some students

I have a teacher using the Kami extension through Google Classroom. She assigned a PDF copy to each of her students through an assignment in Classroom. The students open the document using Kami, but some students aren’t able to use some features. Some cannot highlight, or some can highlight and then the line appears black in a different spot. Why would Kami work for some and not others? How can we correct this issue?