Why can't I adjust my notification sounds and systems sounds on my Samsung Galaxy S6?

[AC Question]

When my phone has the sound on i can adjust the notification and system sounds. But when I put it on vibrate, the sliders to adjust the volume are greyed out. I would like to have it so that when sound is on, all sounds are on, when it is on vibrate, everything is on vibrate, and when it is on silent, everything is on silent. Seems simple enough right? I noticed this problem because, my Groupme and Facebook Messenger Apps stopped vibrating even though it is checked in the app settings. Any ideas?