The first time I tried setting this up, I was surprised to have to add the Discourse app to every channel. I’ll keep a look out for any way of working around this requirement.
Possibly an error message is returned from Slack to Discourse when an action fails due to the app not having permissions to post to/from the channel. If that’s the case, using the error to add a notice to the Channel’s entry on Discourse would be helpful for debugging this type of issue. I’m not sure if this will be possible, but I’ll have a look to see if Slack returns any useful information to Discourse for this case.