Why doesn't strengthening the lessons work?
Sometimes I can strengthen a lesson by completing a time-tested run with 5 or 6 correct answers. But other times, it doesn't happen. I sometimes even complete a non-time-tested run and get it all correct in a row, but it still doesn't strengthen the lesson. Why? What could be the problem?
No idea. Maybe try doing it without time, because I do that all the time, and it never happens.
Sometimes it just does that. When it does, do each lesson in the skill and it will strengthen.
You can check with the user script "DuoLingo skill strength viewer" what happens to the percentage of the words per skill.
Sometimes you have to strengthen multiple times.
It happens also to me, exactly how you describe it.