Course completion can be registered in a number of different ways;
1. Learner visits every screen in the course
2. Sets course completion option ticked in the Assessment Results asset

3. Sets course completion action added to an interactive asset e.g. Button, Image Button  

Troubleshooting

1. "I've set course completion on the assessment results asset but the course is showing completion if the learner fails the assessment"
Resolution: If the Assessment Results screen is also the last screen in the course then the 1st rule of course completion will also apply. To get around this you could add an unreachable screen/topic to you course. This would prevent the 1st rule from being applied and the learner would need to pass the assessment by the required mark in order to complete the course.
2. "I've got two Assessments in the course but only the last assessment score is recorded in the LMS"
Resolution: Only one assessment is sent to the LMS. The course will send the results from the last assessment in the course to the LMS. The scores from Assessment results assets can't be combined and sent as a total score.

3. "I'm using Continuous Scroll topics but topic/course completion is not being recorded"

Resolution: This issue generally occurs when there is minimal content on the final screen in a topic. As the Learner scrolls down the final screen is not registered as active/complete. One way to ensure topic completion would be to tick the 'Set as final screen' option on the penultimate screen of each topic.