How to investigate why students do not do their homework? I teach freshmen courses. The lessons are in the classroom, but I also use an LMS to receive homework, to communicate grades and comments, and to post additional readings and resources.

It is normal to encounter a few students not doing work, but this term, I found about 80% of one section doing no homework, even easy work. To investigate why so many students are not doing the work, I tried these strategies:


I sent private messages to each student informing them that they missed several assignments and that it is their responsibility to inform me of any problems. The students did not reply.
I posted a private survey to the LMS to all students, asking for students to describe any problem they were having with the LMS or homework. None of these students completed the survey.
Seeing no replies after 5 days, I set aside some of class time for a whole-class discussion on this topic, but this made students nervous. In retrospect, realize this was a poor choice.


I did learn through the discussion that some students did not know how to use the LMS and now that we've spend 20 minutes covering that (again).

After my discussion, more homework is coming in, but still about 50% of the class has a 0%.


I do not see this problem in my other sections, nor have I had this failure rate in past years.
A student signed me into their account, so I could confirm that there were no anomalies in the LMS for this section.


I want to identify why this section has so few students submitting work, so that I can make appropriate adjustments to the course, as necessary. How can I investigate this problem further, without making students nervous to explain?
You have been around the block a few times, so I interpret this question to be "What is going on with THESE PARTICULAR students?" And of course I don't know. 

I have run into some scenarios at my large public university which may be relevant:


Recent admission changes which bring in students from more low-income schools with less experience with self-regulated learning.
Changes in course requirements which allow less-experienced students to take the course. 
Limited timing of a different required remedial class made one of my two sections also all remedial. 
A shift to making a class more online also allowed students to fall behind more easily. 


Whatever the reasons, I would also want to know why this results in homework avoidance and how to help overcome this. 

Have you tried having undergraduate tutors ask the students what would help? Your students might feel safer admitting problems to peers.