Why are only some apps showing up in the notification panel?

[AC Question]

A short while ago I stopped receiving notification icons in the panel and lock screen for only some apps (GroupMe). If I am actively on the phone, I see the popup on the top of the screen but nothing shows up in the panel like all of my other apps do (messages, whatsapp, facebook,...). So unless I actually open the app or an on the phone when the message was sent, I will not know about any messages. I checked all the settings and they are all unchanged. I have tried uninstalling, deleting the app and all data, reinstalling and still have the same issue.