On rare occasions, you may find yourself in a situation where you'll watch a whole video lesson through, go to the next lesson, and find the previous lesson wasn't marked as completed. This can happen for a number of reasons, including glitches with your browser, server glitches, etc.
However, this can be easily remedied by manually toggling the lesson completion option via the instructions below.
1) On the course home page, click the Lessons button in the bottom left to bring up the lessons list.