Windows 10 is stopping VST scan halfway through.
 
I recently added Eventide 2016 Stereo Room.dll  Could that be the problem?

Perhaps the splash screen will be behind other windows, and you won't see that you have to click something to proceed. Check the task bar. This happens a lot with iLok-protected software that hasn't been authorized yet.

You should probably subscribe to this forum submission. It is a Microsoft update caused problem! http://forum.cakewalk.com/FindPost/3530846

After is stops what does it do? Any messages? How long did it stop for?
There are periodic stops sometimes that do not necessarily mean that scanning has completely stopped. 

KingsMix
After is stops what does it do? Any messages? How long did it stop for?
There are periodic stops sometimes that do not necessarily mean that scanning has completely stopped. 

Very true. I have plug-ins from a particular company that take forever. When it stops, I start again and it makes it past the plug-in where it stopped, and keeps going...until it stops on another one. Rinse, lather, repeat...the good news is once SONAR knows it's there, this tends not to happen again until there's an update, reset & re-scan, etc.
 
(BTW there is the issue referenced above by pfossi, but that's not necessarily the only cause. I mention the alternative because scanning is working fine here.)

KingsMix
After is stops what does it do? Any messages? How long did it stop for?
There are periodic stops sometimes that do not necessarily mean that scanning has completely stopped. 

I also have this issue and it coincides with the problems caused by the Microsoft update interaction with Sonar. My VST scans used to be quick and flawless. This may be coincidental and unrelated but I've ready repeatedly about VST scans as one of the manifestations of the Microsoft update problem.
 
I hope Cakewalk provides some sort of update soon with a tentative timeline.

This workaround just worked for me:
 
I moved the offending .dll's out of the vst folder and onto the desktop.  Completed manual scan.  Moved dll's back into vst folder.  Everything shows up.  The two offending dll folders for me were Rapture Session and Z3TA+ 2 located in the "Program Files/Common Files/VST3 directory.  I moved the entire folders out...then put them back in after the scan.

it may be worth trying the scan from utilities/ plugin manager rather than via edit/preferences vst settings.
For my machine it hangs unless I scan using plug in manager and yes I am on the release on Windows 10 causing the issues.