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. This is separate from turning in the assignment in Google Classroom.
Students Don't See a Turn In Button
Students may not be seeing a Turn In button. Make sure they open the assignment from Google Classroom, instead of going to it directly on CK-12. CK-12 assignments made to Google Classroom have class and assignment identifiers in them, and if students don't go through GC, then those identifiers aren't there.
Even if they initially go through GC, 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 Google Classroom to get there.
So, after they complete one assignment, they should go back to GC and access the next assignment there.
Students May Have Clicked "Done" in GC, Before (or instead of) Clicking the Turn In Button
Students must click the Turn In button on the CK-12 assignment before marking the assignment "Done" in GC. If they are marking the assignment "Done" in GC first, this could cause the problems you are seeing. If you think your students may have clicked these in the wrong order, please have them try the following in GC:
1. Click the "Open" button
2. Click "Unsubmit" button
3. Click on the assignment link to open the practice and then click the "Turn In" button.
The student's report should get attached at that point.
Students may be completing assignments in the wrong class
This can happen if you create your class in Google Classroom, 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 Google Classroom, click here.