From the CNN report:
"...drill down into that number and you'll see signs of a stirring discontent on the left," says CNN Polling Director Keating Holland. "Thirty-eight percent say they disapprove because President Obama has been too liberal, but 13 percent say they disapprove of Obama because he has not been liberal enough - nearly double what it was in May, when the question was last asked, and the first time that number has hit double digits in Obama's presidency."
It should also be noted that complaints of the President not being liberal enough were not just relegated to liberals themselves. 14% of self-described moderates who disapproved of Obama cited the same reason, and even 9% of conservatives.
The poll utilized phone interviews with 1,009 adults conducted from July 18th to the 20th. Read the full results here.
Update: An earlier version of this post stated that 16% of moderates disapproved because the President hasn't been liberal enough. The correct number is 14%.