Being able to use your device somewhat hands free by initiating the "OK Google" hotword is one of the better uses to come to the varied range of Android devices which support the feature, but those who owned a Samsung Galaxy S6 and Galaxy S6 Edge may have been dealing with a bug which caused the command not to work. The earliest reports about this problem surfacing seem to be as far back as April 15th when the thread was started on the Google Product forums about the issue, and it seems that according to some Google employees engaging in the thread with users, they're stating the cause of the bug was from some Google employees doing "compatibility work" on the device.
The good news is that the bug seems to be fixed, with Google having begun to push out a patch for the problem to bring the voice command back online for users. It's worth noting that this bug only seemed to be affecting people's ability to utilize the OK Google voice command from any screen, but not the ability to use voice to issue commands from within the Google Now screen, so if you're still experiencing this issue on your personal Galaxy S6 or Galaxy S6 Edge, navigating to the Google Now screen or opening up the Google app and tapping the mic button should allow you to issue voice commands for whatever tasks you would normally use the OK Google hotword for.
Although the issue is fixed, according to some posts in the forum, the ability to access the OK Google command from any screen is now only capable when the device is charging, whereas reports from users state that the "when is charging" requirement was not there prior to the bug taking place. Whether or not the command will be reverted back to a state where you can use it anywhere without the device having to be plugged in is unknown, but at least now the issue is fixed and users have access to it again. The bug fix should be rolling out to users of these devices soon, but realistically it could take the next few days before people start to see it on a mass scale.