Vibrate mode very unpredictable on Galaxy S6

[Coach88]

I've recently upgraded from the Galaxy S4 to the S6, I've been having issues with the notifications. The one thing that remains constant is that it's very unpredictable. My previous S4 was acting quite the same. Yesterday, since I had the time to experiment a little, I noticed that even though I was in Vibrate mode, my phone didn't vibrate when a txt came in. I noticed the incoming txt because my screen lit up. This happen a couple of times. The active app on my phone at the time was my TXT app. When I changed it to the main screen or a different app, my phone vibrated when a txt came in... I thought I had finally figured it out! Wrong!!! Later in the evening, I missed a call... still on vibrate, but I never noticed the notification. I was in the same room while the call would have come in. This morning, not wanting to be bothered by a txt message while trying to sleep, my phone was in the txt app but every txt that came in... it Vibrated!!! Change of behavior not a result of a change in settings since I didn't touch any settings... Doesn't seem to be any rhyme or reason to this mess.