I don't know if this has been discussed or noticed by CC Photoshop users, but when using the curve tool or levels tool, the on image adjustment tool does not seem to be as sensitive in differentiating tone variations in the image. It's a little annoying. In CS5 it was very sensitive in distinguishing tones.
I run a Mac OSX Yosemite with CC Photoshop release 2017.0.0
I have not noticed any change in the way Levels or Curves works.
What type of image are you using them on? Can you show an example? Keep in mind that mid-tonal adjustments have no effect on "binary" images such as line art, flat-field graphics, or halftone images. They will also have less effect than "expected" on very noisy and grainy images.
I figured out the problem. there is a pixel radius adjustment that can change the point sample from 3x3 pixel average all the way up to 101x101 pixel radius average. that is why it was not differentiating between tones. it was set way too high.