Hello!
Did you try the "Find a Quest" already? (It´s the little map with the magnifying glass above the quests on the side or at the bottom of your Questlog) Maybe you missed a quest on a Spot which isn´t visible at first.
Also what might happened is that you´re close to the next level and you´ll have to grind.

Not really fun but I had that issue in the past. Did you join an Exp Guild? It gives you some more Exp!
Hope that helped already if not feel free to ask again!
Greetings