The meter will not be able to be filled up all the way, if the required skill levels haven't been met. Academic potential moves up as skills are earned. Each major requires a different number of skills. Typically during the first semester, a skill point is required in three areas related to a Sims major.
To avoid spending all of a Sims' time in college just earning skills, plan ahead and earn the skills they need to get through college during the teen years, of even when they are children.

