Viber - issue with notification message

[cherryME]

Since a while I keep missing messages on viber. The message shows up, when I open the app, but before that I don't get a notification message, that I have a new message. How can I repair this issue?


P.S. I have Lumia 920, WP 8.1.
and the notifications are turn on in the viber settings and the app is also allowed to run in the background.