Apple monitor doesn't go to sleep with power saver setup

I am using the 24in Apple Cinema Display (mini-DP) on my Ubuntu 16.04.3 with Thinkpad P50. The problem is the display doesn't automatically turn off or go to sleep even I have set up the energy saver options for sleep and turn off times. I didn't find the solution on the internet. If you have any idea to check, please let me know.

Thanks!