you click on start, then logout..don't have to exit but just logout then it'll ask for account/pass again, blah blah. this problem usually happens when you're in a party and the party bugs (disappears) nobody will get XP until you relog or the party leader relogs and resets the party.
tho this can also happen if you're in a party and have a person too high of a level (or too low) near you, anything with a bigger gap than i think 19 levels somebody won't get xp