Do your students pay attention when you speak? I teach in a classroom where students sit at desks with dual monitors in front of them. The students face my desk at the front of the room, I cannot see their screens. Due to the height of the screens (set by our Accommodations staff) I can hardly see their faces when I am sitting down.

When I lecture, I use a projector to show one of my screens for programming examples, good links, videos, etc. I also project pages of the textbook and other printed materials. But I am not used to students sitting at desks with computers: when I was in school we didn't have them, and in college we had standard lecture rooms. There was nowhere else to look, really, except at the only moving thing in the room - the Instructor.

Often, when I talk, the students are looking down... At their notes? The book? Something else? I can hardly see them behind their screens anyway (must they be raised up all the way?) I try to present vital points on at least 3 different days, but sometimes nobody "gets it", or only one or two do. Is it my teaching, or their attention? Do you experience this also and if so, what do you do? I have trouble giving firm directives due to my personality tendencies.
Ask them to switch their screens off.

They don't like it if you do it too often or for too long but it's surprisingly effective, saves you having to repeat yourself unnecessarily and forces you to try to make your instructions clear and concise. If you can't see their screens you can always tell by the glare from their screens on their faces who hasn't followed instructions.