Have you looked at the task manager to see if anything is running in the background? When I was testing out shadow play, ge force experience thought it would be cool to keep my entire system at 100% usage while the computer was supposed to be idle.... it was a really easy fix.

Also, what is your monitor refresh rate? That may be having a minor effect.

I just checked the nvidia processes nothing was eating up much, 0% I closed them all down to see if it would affect the clock. Nothing changed though. My monitor refresh rate is 144Hz, would this cause it? It wasn't a problem before :(

just tried going down to 120hz, 100, and even 60. nothing changed :(

thanks for the input guys! didn't even realize i had Kboost on I don't ever remember turning it on! that was my problem. 
 
Thanks to everyone for the help!

Lol. At least it was an easy fix.

Hi Guys.
I just received my 1080 Ti Black edition and was having the same issue.... no downclocking...stuck at 1556 and 5508...so I found this post and noticed that K-Boost was enabled by default...
Tried to turn it off... got a message saying turn off 3D applications first.... but I wasn't running and 3D apps...... I thought....
 
SO...... If any of you have a Logitech mouse and / or keyboard and are running Logitech gaming software ....disable it before you try to turn K-Boost off....
Shut down Logitech gaming software.. turned off K-Boost... Card now downclocks properly 
I remembered that the same Logitech software should NOT be running when installing drivers or Precision X as well.