This is possibly because your channel(s) isn’t (aren’t) authorized anymore. The admin can check if that’s the case by going to ‘Manage settings’ and then ‘Channels’. Find the channel that’s in the stream that isn’t loading any new messages and check the state of the channel. There are three states: “On”, “Off” & “Unauthorized”.
To restore the authorization, please ask your admin to add the channel to Media Injection again. The admin has to go to “Manage settings”, “Channels” and click the plus sign in the top-right corner of the screen. Next, just choose the corresponding channel and log in with the correct account. You will receive a notification that the channel already exists. That's right, but this way the channel is authorized again. Note that the channel doesn’t have to be deleted from MI. Just re-adding is enough.
Please make sure your ad-blocker (either in-browser or separate apps) is turned off. Otherwise, you won't be able to (re-)add channels.