I don't know whether this is caused because of a wrong setting in the admin console or it is a bug. We are using Jive cloud (public) and using an add-on where we often need the app-mention (! App) functionality.
Since a couple of days this functionality is not available when I add an ! to a doc nor on the bottom of the text field. I couldn't find a setting in the admin console or a similar case in the JC or in the Jive documentation.
Sorry to hear the "!" feature is no longer appearing in your instance. As a next step I would recommend opening up a case in your customer group here in the Jive Community so that the next available support engineer can assist with further investigations related to your specific instance.