Rise: Continue not advancing to next lesson...for 1 lesson only

Hi there,

This is my first Community post, so hello!
My question is on the Continue button.

I have created a course with 8 individual lessons.
A Continue button (some labelled "Let's Go") rests at the bottom of every lesson so that the learner flows straight into the next lesson.

Upon receiving feedback from a user, lesson 6 stops at the end. 
To advance the learner must click the lesson name at the bottom.

This is confusing for the learner due to its inconsistency. This learner in question was unable to advance as had simply been pressing continue in all previous lessons.

Any ideas what is happening? The settings seem pretty straight-forward.