You’ve used all your EXP cards? And found every optional quest in the previous maps?
On a perhaps somewhat related note, I had my character in a party since level 30 or so and never took them out of it. When I quit it yesterday, I found that my EXP gain increased dramatically. I was the only one in the party, but it seemed like it was eating my EXP, as well the EXP from my cards.

The item drop is another thing though