There may be a number of reasons for this:
Students Aren't Clicking the Turn In Button
Make sure your students click the "Turn In" button at the top of the CK-12 lesson. That is what writes their score to your reports.
Students Don't See a Turn In Button
Students may not be seeing a Turn In button. Make sure they open the assignment from Schoology, instead of going to it directly on CK-12. CK-12 assignments made to Schoology have class and assignment identifiers in them, and if students don't go through Schoology, then those identifiers aren't there.
Even if they initially go through Schoology, but then use the arrows in the lesson or other navigation to get to a different lesson that is also assigned, when they get to the next lesson they won't see a Turn In button. Why? Because they didn't actually go through Schoology to get there.
So, after they complete one assignment, they should go back to Schoology and access the next assignment there.
Students may be completing assignments in the wrong class
This can happen if you create your class in Schoology, and also on CK-12. Click here for more details.
Where are those grades, anyway?
If you are not sure where to find grades for CK-12 assignments in Schoology, click here.