Slack Channels and Restricted Access
Slack lets you restrict who can post in a channel. Every workspace has this on #general, and Plus and Enterprise plans can set it on any channel. A restricted channel blocks the Manifestly bot along with everyone else, so notifications silently fail to arrive.
The fix
Add the Manifestly bot as one of the people allowed to post in that channel, in the channel's Slack settings. Once it can post, notifications behave normally.
How to recognise this
The symptom is specific: everything is configured correctly in Manifestly, the channel is selected on the workflow, and nothing ever appears in Slack. No error is raised in Manifestly, because as far as it is concerned the message was sent.
If notifications work in one channel and not another, posting restrictions are the first thing to check, ahead of anything in Manifestly.