You were using past sessions to calculate your average. Perhaps a setting is set wrong.

In this example, the league was using last 10 weeks of a previous session to calculate the average at the beginning of the league. But once the 10 weeks of this session were played, the system continues to only use the last 10 weeks.

If you want to use all of this season once you've played 10 weeks, check to make sure it is set right.

Go to Stats/Standing Grouping (Menu > Manage > Statistics/Standings > Stats/Standing Grouping)

1. Go to Players,
2. Select the League, 
3. Pencil to edit, 
4. Advanced section.

5. Verify if your box is checked or not to Use all weeks of this season for average.