At level 19 before you can level again you have to pick a final class and complete its quest.
For instance if you were a fighter till level 10 then you picked crusader it is now time to do your class quest so you can become a Paladin. If you were a summoner now you become a conjurer. if you were a Enchanter now you become a Illusionist.
If you don't do this quest you can never ding 20. I believe you can get to +220% exp but no further.
Being I am from Freeport I have no clue on where you need to go for this step.