Issue
iOS users on 15.0-15.4 are not receiving messages in real time due to an Apple issue.
Environment
iOS 15.0-15.4
Solution
This is resolved in iOS 15.5+.
If you are still on iOS 15.0-15.4, we encourage you to update to the latest iOS version.
Outcome
Alternately, you can "pull to refresh" messages as a workaround.