Sensei plugin - Single Course Page doesn't show lessons

I am struggling with the Sensei plugin on my website. I love it, but cannot make it show the list of lessons included in any course.
It shows the header "Lessons" but it is empty below even though I have many lessons included.
I am sure this is not about the settings but something in the coding.
The plugin demo page looks like this, and this is exactly what I am trying to achieve:
Any idea would be appreciated.