Im not quite sure what the problem is but sometimes when im trying to do a preview of a new change the pic will stay the same until i click on (or do)something else.
For example, If you have two layers and you click on the eye to make the layer disappear the pic will remain the same until I actually do something else ( such as use the hand tool and move the pic). As soon as I do something such as that the pic will change to show what it would look like without the layer selected. I hope that makes sense.
so how do you fix that to give you that instantaneous result?
I was adjusting the white balance in curves and it wouldnt show the change until after I did something else. Which is very annoying.

