It may be , because of your settings.
My problem solved when I changed the settings.
# ''Go'' to ***settings***.
# Then ''click'' ***Notifications***
# There will be a list of apps which recently sent notifications. Found ***Android*** ***System*** among them and ''open'' it.
# ''Scroll down'' and look whether the option ***“ Apps are using mobile data “*** is ''on.''
That was my problem. After I ''turned it off'' everything became alright ?