Have you completed the Slack integration settings for each calendar, in addition to connecting Slack under Team Settings?
If you have, check the following points again.
Is the Channel Correct?
Is the Channel configured for the calendar correct?
For a public Slack channel, select the Channel from the drop-down list.
If the Channel does not appear in the drop-down list, confirm that it is a public channel and that the correct Slack workspace is connected to the team.
For a private channel, enter the Channel manually.
The value you enter must exactly match the actual channel name.
For Private Channel Notifications, Has the TimeRex Bot Been Invited?
To send notifications to a private Slack channel, you must invite the TimeRex bot to the private channel in advance.
If you have not invited it, use either of the following methods.
Method 1: Invite the TimeRex bot by mentioning it
In the private channel, enter @timerex to mention the TimeRex bot.
Click Add to Channel.
Method 2: Add the TimeRex app from the channel settings
Open the private channel settings.
On the Integrations tab, click Add an App.
Search for TimeRex, then click Add.
Is the Correct Slack Workspace Connected to the Team?
TimeRex connects to a channel in the Slack workspace connected to the TimeRex team.
If a different Slack workspace is connected, disconnect Slack under Team Settings, then connect the correct workspace.