You're most likely over level for the enemies in the area. If you mouse over an enemy (where you'll see the enemy's name, health, etc.), you'll also see a number, ranking the enemy. The color of the number shows how much xp you'll get. If the number is gray, you get nothing. Red (much harder than your level) gets you the most xp, then orange, yellow, and green - which gives the least (aside from gray, of course). Missions are rated in the same way (Although they usually give an insignificant number, such as 5 xp, instead of nothing).