Make sure you have the checkmark above the completion checklist. If the video is not showing completed, make sure to watch all the way to end of the video. For the lesson, make sure you scroll/swipe all the way to the bottom.
If the lesson has more than one video, make sure you watch both videos.
NOTE: Sometime both videos might not appear on your mobile device. To watch both videos and complete the lesson, go to the apps website.
NOTE 2: Some courses and video series only allow you to see one lesson per day. Make sure to wait 24 hours from when you completed the previous lesson to see if it unlocks.