Why won't my apps use background data? (Even tho Background Data IS enabled)

[AC Question]

My Galaxy S5 randomly started giving me issues where my apps don't seem to be using the background data, even though the Background Data and Sync is turned on. For example, I'm not getting emails when they actually arrive, I only see that I have new emails when I actually open and refresh the Gmail app. This is also a problem with any other apps that usually give me notifications--I'm not getting sports scores, Facebook messenger notifications, etc.. unless I actually the app.

Any suggestions? I've looked through every setting possible.