What other factors have changed since you first gave the course? E.g. the student evaluation questionnaire, the pandemic, the student cohort. And during the course, did the students seem to enjoy the course more or less, or find it easier or harder? (I think you could make this two questions - why did the evaluations get worse, and should I go back to the old way - or maybe not, because probably one of them would get closed for some reason.)

@toby544 of the course the group of students is not the same. The main changed factors are those which I thought are improvements of the first version of the course.

I meant did their characteristics change in some broad way, not were they exactly the same people. Are you sure the pandemic was not a major changed factor?

@toby544 Last year, for example, I gave the same course online (due to the pandemic) and since it was the first time I teach that course online, I think the quality was very bad and the students commented objectively about the quality and I agree with them even before even reading their comments. However, their scores were very good compared to this year, where I improved the quality significantly (at least from my point of view). This year, the students had very different opinions and the scores they gave are relatively bad.

"I believe I have to explain on a higher level." It seems what you did was on a level too high for your students. So, either do it on a lower level or get better students. Perfect instruction is no good if its level does not fit the students.

Has the difficulty/sophistication of the course creeped upwards? I find that sometimes that as instructors become more confident in the material, they increasingly stress the more difficult aspects that they find interesting, while the students are still struggling to learn the basics.

Could there be a communication problem? Especially the example “we had to search by ourselves to understand” points to this in my opinion. Did you tell them that that was expected? In many courses all the material is simply given to the students in bite-sized chunks, which they may have come to expect. I can’t imagine a student leaving this comment if they were explicitly told this was part of the course. If there are one or two more of these cases where something that’s obvious to you was left unsaid, that might explain the evaluations despite the impressive improvements.

I would have hated your course. Videos are boring and don't convey information. Having everything "automated" means less human interaction. The "hybrid" thing is a big turn off. Slides just mean you took my notes for me, so I'm pushed into being a more passive learner. As a student, I wanted to feel like the prof and I were exploring together. I didn't want him to hand me a map and compass and shove me down the path.

Is your teaching also being evaluated in other ways, or only by student evaluations?

Following on @11684 's comment, the responses of the students who said that they needed to watch videos, before they understood the material, are providing valuable feedback. Regardless of what you think of 3brown1blue, it seems to be an objective, empirical fact that it filled a pedagogical gap in your courses that is no small matter, and that you should not ignore. Instead of dismissing these responses, perhaps you could start a discussion with those students about what difficulties those videos helped with, and how they did so, with the goal of modifying how you cover those topics.

@Younes How well are your students learning, based on your evaluation of their homeworks, exams, or other written work? Has this gotten better or worse over the last four years?

"I master the topic very well now", "this makes them of course much more difficult but better to learn". So, is your course more difficult now compared to before? if so, the increase in quality otherwise might be offset by the increase in difficulty.

The reasons you give for how it is better provide some insight. (1) I agree this helps, although students usually cannot see it. (2) More difficult or realistic does not at all mean better for learning. (3) Having a transparent organization is almost irrelevant to the students. It mostly allows an expert to quickly see what the class covers. (4) Do all of the "etc." points also only concern your perspective and ignore the student perspective? I was surprised that none of these points involved positive student reactions or increased student attention or increased class attendance or... etc.

The quality of your course is essentially orthogonal to what students think of it. The student body, on mass, likes being spoon fed and given good grades. That's not what they learn best from.

Thank you all for your comments! But @B.Goddard did you read my entire question? Well, two things, I feel you missed: 1) I am comparing their evaluation based on the previous year's evaluation, where I improved this year's course w.r.t the feedback of former students. Therefore, this improvement is not based on my perspective only. 2) I expected my students to use other materials to grasp the topic and I encouraged them because the topic is very very vast and complex and we cannot explain everything.

3) The course is very very interactive compared to ALL courses even before the pandemic. My reason is that the tutorials used to be given by PhD students who usually do not master the topic entirely and in ALL courses, the tutorial does not exceed 40 min (although 90 min is allocated for it). This year, I decided to give the tutorials myself which takes 90+ min covering from basic linear algebra to very high-level examples.

4) This year I had 340 students compared to 150 last year ad due to legal reasons, this master program is not selective. Thus, any student (also internationals) who fulfil some basic conditions are admitted.

5) One main change compared to previous years and also compared to other courses is the workload on the weekly assignments which I increased by a factor of 2~3. However, I made sure that an average student can solve 80% of it in the time (s)he is supposed to spend for the course (8 hours a week). How did I do it? The assignments are prepared and corrected by good former students. The strategy was that they solve each other assignments and have to finish it in 3 hours.

In my experience only a handful of students appreciate the quality of the material distributed. Many students are happy if you give them some generic publisher slides, with all the slides you don't discuss removed. Sad but true.

Could you explain the sentence "One of their common reasons, is that they could understand some topics better when they watched youtube videos."? Does it mean that they failed at your course because they couldn't find youtube videos on that topic, or does it mean that they found your teaching material to be even worse than a youtube video?