Rebecca Lee Kit-ying, an instructor in biomedical science at Chinese University, is all too familiar with this. "I was usually greeted by silence when I asked if anybody had problems understanding what I had just taught," she says.
University lecturers can be faced with classes of up several hundred students, so they can't quiz them all individually.
So how do they find out if they are getting through to their class?
For Lee, such dilemmas have largely faded. Last month, the university introduced a learning system that combines cloud-based computing with mobile technology that helps them assess students' grasp of material - and encourages their participation.
Tapping arrays of servers over the internet to boost capacity for storing, processing, and accessing vast amounts information, cloud computing is enabling lecturers to keep close tabs on their students' performance. That is why universities here have increasingly turned to cloud-based systems.
The new system at Chinese University, called Mobile Learning @  CUHK, is the work of associate professor Paul Lam Lai-chuen and his colleagues from the Centre for Learning Enhancement and Research. The fruit of three years' work, it supports instant responses from students, lecture recording on a video platform and videoconferencing in chatrooms.
This facilitates interaction between students and tutors outside of the classroom. It's a boon for teachers, Lam says.
Lee can testify to that. At the end of a recent lecture to 50 first-year students on the functions of sensory organs, she quizzed her class on the physiology of the eye. The students replied immediately. Poring over notes in their laptops, they keyed in answers on their smartphones. They showed an impressive display of digital dexterity as they plugged into the U-Reply platform that is part of the mobile learning system
More importantly, they gave Lee an instant picture of what they had learned. Their replies were converted into a bar chart, and the distribution showed that most got it right.
The system encourages participation, Lee says. "Even those who are shy, or seated at the back of the theatre, don't mind texting their responses via smartphones."
Previously, lecturers giving quizzes handed students unwieldy "clicker" devices to input their answers. The devices limited the format to multiple-choice questions, and had to be distributed and collected from students each time they were used, Lam says.
With the uReply system, which supports both multiple-choice and written responses, teachers can now get an exhaustive report on the participation rate for each question, and answers from all students.
A number of teachers have become enthusiastic adopters, Lam says: "One teacher has gone so far as to require all students to input their ID and name before the question sessions, so that answers can be counted towards their overall assessment at the end of the term."
The new system also supports graphic images from doodles to diagrams through its uDraw platform. This allows students to make instant sketches, which is useful for architecture courses, and music, where students have to write scores.
Lam says the university's huge investment in a cloud system and Wi-fi network has made it possible for teachers to access students' responses at any time, anywhere.
"Before, teachers had to use memory sticks to save students' responses as the data could only be retrieved from the computer in the lecture hall. Now, you can get the data any time you are online."
At Baptist University, the computer science department is embracing another cloud-based system called maXit. This is designed for video capture and image analysis.
It was jointly developed two years ago by Beaconwall Ltd in Hong Kong and maXit Systems in the US for the Hong Kong Sports Institute, which wanted second-by-second analysis of athletes' performances.
"They asked us to develop a system for fencing and martial arts training to help them prepare for the Asian Games," says Beaconwall director Patrick Hung Chak-kuen.
"In a training session, two cameras are placed on the left and right of fencers to record their movements from different angles. In the past, they would have two people to press the buttons manually.
"But there was always a time discrepancy of around one-tenth of a second, which made precise analysis impossible. They also lacked a system to store the images for easy retrieval," Hung says.
The maXit system can also churn out image-based reports of each bout. The system quickly drew interest from educational institutions, including nursing and medical schools in Canada, and the US. They saw its potential as a teaching aid, Hung explains.
"They need to record students performing medical procedures from different angles. Using the recorded images, a teacher can see if a student is applying a needle in the wrong spot."
Such detailed attention to physical performance is not required of computer students. But William Cheung Kwok-wai, associate head of Baptist University's department of computer science, says the system can help improve students' presentation skills.
"Many of our students have very good technical knowledge, but lack communication skills. They can't explain solutions to technical problems in a concise and clear manner to teachers." That means they won't be able to do that with potential employers, which will affect their careers, Cheung says.
In the past, he would video student presentations, load them into a computer, and then copy the video on to discs for individual students. They would then review the footage together.
"I marked the parts during the presentation where information was misrepresented. Then I got the students into my office to tell them what went wrong, while replaying the video. Students found these after-class sessions very helpful. But it took up a lot of extra time.
"With maXit, teachers can respond instantly throughout a presentation via their iPad [or other tablet], pinpointing sections where improvements are needed.
"Each student can get a report after the presentation listing the teacher's comments at different points. The system also allows teachers and students to retrieve the video and comments easily online."
Which all goes to show, there are times when it's an advantage to have your head in the clouds.