"Next" button does not work

When I finish a lesson (instructions included) and I click on the “Next” button to go to the next lesson, nothing happens. It seems the “Next” button does not work.

I had a similar problem with the JavaScript lessons. The “Next” button wouldn’t work despite running the program and completing the instructions. I found that the “Next” button tends to light up and work after pressing the “run” button several times.