Adjustment with levels in Photosohp CS6

Something has gone wrong with my use of adjustments of brightness using levels in Photoshop CS6.

Following is the method I always used successfully until it would not work today:-

Step 1 Click on IMAGE
Step 2 Click on ADJUSTMENTS
Step 3 Click on LEVELS
Step 4 Shift the black arrow to the required brightness/contrast desired.
Step 5 As the black arrow is shifted, the picture on the monitor darkens
according to the desired level.

Problem: The picture does not darken at all.

Hope I am making myself clear, and I send my thanks in advance for your help.

Finally, sorry I don't know how to enter in the above message a screenshot of the dialog box showing the black arrow.

Joe