There are two potential reasons: 

1. In order for a lesson to be marked complete you must complete all the activities - this includes watching videos, answering all Knowledge Check quiz questions, and doing the labs related to the lesson. If you have not done all these activities the lesson will not be marked complete.

2. When you log in to Pathstream we calculate your progress through the lessons to mark them as checked. It may take a few moments for the checkmarks to load. This does NOT mean any of your work was lost.