Upon reaching level 60, you must increase your level caps through potential development. Each job's potential development is independent from each other, although some requirements may only be done once to unlock all classes (such as getting titles rather than collecting items). You can see your progress in the status window (Ctrl + A and click the Attributes tab). The Potential Development Instructor is located at X:155 Y:172 in Eversun City. Speak to him to unlock a level.