I have this exact same problem. It was fine on windows 8.1, then I decided to fresh install windows 8.1 and it mucked it up. I thought windows 10 would address this issue but it hasn't.
I've rebuild my index numerous times, stop windows services, restarted, clean fresh installed windows 10. NOTHING WORKS!
Its more frustrating because i wanted it to just index my second hard drive and it takes a while for it to complete. However it still doesn't work.
Also windows search now won't even show basic setting when searched for example Sound,Network, Device manager.
Side note: sometimes it'll work out of no where but the majority of the time it doesn't.
THIS IS VERY FRUSTRATING!
Edit: Also what I found out is that when command prompt sfc /scanow I get this error
C:\WINDOWS\system32>sfc /scannow
Beginning system scan. This process will take some time.
Beginning verification phase of system scan.
Verification 100% complete.
Windows Resource Protection found corrupt files but was unable to fix some
of them. Details are included in the CBS.Log windir\Logs\CBS\CBS.log. For
example C:\Windows\Logs\CBS\CBS.log. Note that logging is currently not
supported in offline servicing scenarios.
Edit 2:
I have tried dism /online /cleanup-image /restorehealth but it doesn't fix the issue, I get this back.
Error: 0x800f081f
The source files could not be found.
Use the "Source" option to specify the location of the files that are required to restore the feature. For more information on specifying a source location, see http://go.microsoft.com/fwlink/?LinkId=243077.
The DISM log file can be found at C:\WINDOWS\Logs\DISM\dism.log
Edit3:
I fixed the sfc /scannow situation by entering these commands into comman prompt I found this worked for me:
Open an elevated command prompt, right click and select open as administrator.
While still in the elevated command prompt, copy and paste the command below, and press Enter.
Dism.exe /online /Cleanup-Image /StartComponentCleanup
While still in the elevated command prompt, copy and paste the command below, and press Enter.
Dism.exe /online /Cleanup-Image /StartComponentCleanup /ResetBase
Exit command prompt and do a restart after you have restarted Open elevated command prompt.
Dism.exe /online /Cleanup-Image /RestoreHealth
Edit4: I don't know if these things were/are related to each other they are just problems I encountered at the same time. Windows search still isn't working correctly.
Edit5: While using the search to search for sound, It doesn't show the sound setting, however if i click the 'Mystuff' icon it brings up more search options including settings related to sound. So it seems its not prioritising the search results correctly.
Edit6: I seem to have encountered more errors during sfc /scannow. I'm giving up slowly.
Edit7: I had no choice but to reset windows 10 by holding shift when clicking shutdown in the start menu, this allowed me to reset and install windows 10 (not a clean install). This fixed my problem with gettting the search bar to work correctly. Now i'm to scared to index my second hard drive because I feel it will f*ck it up again.