My problem is related to notifications also but for me, at the moment, it's the opposite... The one thing that remains constant is that it's very unpredictable. I have the Galaxy S6 and 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.