This question is similar to the unresolved issues discussed by the OP and another commenter here:
When person A sends a message to person B, the mattermost client for user B does not show the message until user B clicks away from the channel and returns to it.
Steps to reproduce
Messages should send and receive automatically.
When standing beside a coworker on the same network, or when on the phone with a coworker on a different network, we can test the sending of messages and see that the messages don’t come through until we click away and return, or refresh the page.
This image shows a chrome browser developer’s console after letting it sit for some time (not necessarily waiting on a message, but sitting on the same page in the background for a while).