It's possible a bug in the driver is causing an intermittent failure. I've now seen reports on this forum about the feature both working and not working on 700 series Nvidia GPUs.
You can try the the following to clear Camera Raw's memory of a past GPU error and make it try again to use the GPU again.
1. Quit the applications (Bridge and Photoshop for Camera Raw, or Lightroom Classic). 2. Look in the following folder:
If you do not know how to find that folder, you can click the Windows icon, type in Run, hit enter, paste that path into the Run dialog box's text field and click OK.
3. Look for a sub folder with the name of the application not working: "Adobe Photoshop Camera Raw" in your case. If you see any files with names beginning "TempDisableGPU" delete them. (Or delete the whole subfolder.)
4. Then restart the application and see if you can use full acceleration again.
You may want to check here for a new driver: Drivers | GeForce . I've seen reports on this forum and other that people sometimes find a newer driver here even though update software on their computer says they are up to date.
I had the same problem on my Win 10 system with a GTX 1060 6GB: initially it worked bot in LR and BR, but after an error message ("BR found a problem and disabled GPU support" or the like) it did not work there anymore. Your solution did the trick. Thank you!