Google Cardboard Icon not displaying Samsung Android Phone(s) in Youtube App I am unable to get the cardboard icon to display on any 360 videos in youtube. I have Cardboard and Google VR Services installed on my Samsung S5 (just factory reset it, no other apps). 

Youtube is updated to 12.15.55. The cardboard app works fine as expected (responds to movements of the phone). 

When watching a 360 video the cardboard icon is not available and the video just shows in "flattened" mode. It is running Android 6.0.1
All the other threads I have found on this have been older and deal with updates to YouTube of the OS which I think should not be my issue?

I am having trouble with the same on an S3 too, but thought that might be because it is older.
You could try these methods to resolve the issue:


Logging out of Google account in the Youtube App


Some users suggest a workaround involving logging out of the Youtube App to resolve the cardboard icon.



Here is one user's explanation: 


  I logout my google account in the youtube app, the cardboard icon
  appears and can view the video now in 360°





Fix resolution of phone.


If you happen to have changed the DPI on your phone, try reverting back to regular resolution.





3. Uninstall Youtube Updates


Since the cardboard icon used to work fine in older builds of Youtube, temporarily downgrading the Youtube App in the meantime until the issue is resolved.

Many issues feel the issue has to do with the Youtube App itself :



  Only appears to be a problem with YouTube. Cardboard apps and other
  videos I have tried work fine.


Also; 


  The cardboard icon disappeared after last update and it was working
  perfectly in previous version.The cardboard VR apps are still working.





Other issues: Incompatibility


Your device may not be marked as compatible with Cardboard. If your device isn't compatible then that explains why you don't see that option. 



Hope this helps

References


YouTube Help Forum: No cardboard icon displayed in updated Android YouTube app
No Cardboard icon in YouTube app