Cannot navigate to next item on lesson

I’m trying to do my lessons by I can’t navigate to the next item on the list, out of 5, using the arrows on the side of the screen.
I can navigate to some of the itens on the lesson clicking on the small icon at the body of the screen, but not all of them.
Has anyone had this problem yet?

Could this be a problem with the CONVEX or CONCAVE radical? Without them the lessons goes well.

After messing around a bit with the extensions that sort levels and types (sorting quite a few times), I could get the arrow to move through the lessons that included the CONVEX and CONCAVE radicals.