I understand you're unable to see comment notifications streaming to your Slack channel.
First, let's troubleshoot this together:
Is your Figma profile recognizing that your account is integrated with Slack? You can check your connected apps here: Regularly review your connected apps
Has the Figma app been added to your Slack workspace, and is your Figma account connected to Slack? You can find instructions here: Connect your Figma account to Slack
Could you confirm that you've navigated to the Figma app channel and typed on or start in the text field to turn notifications on? If you've already done this, could you try typing off or stop in the text field, then typing on or start again a few times?
Please check your notification settings by typing /figma settings in the Figma app to ensure everything is properly set. You can find more details here: Manage notification settings