Photoshop :: Levels Adjustment Layer / Drop-down To Choose Different Channels

I am using a CS6 trial on Windows 7, I can get more specs later but I can't remember anything else about my operating system (on a different computer at the moment).
 
I have noticed sometimes that when using a Levels adjustment layer, the drop-down to choose the different channels (red, green, blue) is grayed out, and I cannot click on it to choose the different colors. The numbers in the input & output boxes are also grayed out and I cannot type in them. The only time I have noticed this happening is directly after cropping an image. And then when I go to work on a different image and add an levels adjustment layer to it, those things are still grayed out. I have to restart the program to get it to work properly again.If I add a Curves layer, nothing is grayed out, everything works fine; it's just Levels.