Mobile notifications randomly not working

While using text messages on Discord, notifications randomly but often don't work. Normally I get a buzz and the notification light flashing, but even then the light only flashes for a couple od seconds and then stops. When I receive SMS or FB MSN message, the light will flash until I read the message. 

Most of the time there is no notification at all. I have to manually wake up my phone and take a look if there is any new text in the Discord chat. 

I'm using Android but I have heard of the same issue on iOS. My settings all look ok and notifications should work, but they only do sometimes and even then only for a short time.

I didn't see any difference if PC app was on or off, if mobile app was open (screen off) or closed, it's completely random, when it works or not.