Because the Progress by Skill view is sorted by mission, this view only shows skills that are part of a Mission. There are cases where exercises are part of math content but are not part of its mission.
For example, the "Number of solutions to a system of equations graphically" exercise is part of the 8th-grade math content but is not part of the 8th-grade math mission. For this reason, it is deliberately excluded from the Progress by Skill view.
If there is a skill that is not appearing in your Progress by Skill view and you want to know if this is because it is not part of a mission, you can access the Mission page of the subject and hover over the skill boxes to determine whether it is available in the mission.