Answer
Application Search depends entirely on Search Domains being properly enabled and synchronized underneath it — if results aren't showing up, first check whether the relevant Search Domains are actually enabled. Going into a specific Search Domain's own detail view also shows useful diagnostic information, like when its text index was last rebuilt, optimized, or synchronized — if that information looks stale or the domain isn't enabled at all, that's almost certainly why Application Search isn't finding the data you're looking for, rather than anything needing extra configuration on the search feature itself.