Slack: How to prevent Authentication requirement every 24hrs

I have a client who has gone through the process of setting a Send Message Slack module in their scenario. We have had a bit of back and forward interaction on their setup, and by the looks of things they have done all that’s required.

However they have to authenticate every 24hours or the scenario stops working, what would be a likely cause step/setting etc that will be doing this?

I’m after a quick response that I could go back with before adding screenshots, since I’ve seen their setup and are pretty happy they followed the correct steps.

Their channel is set to private.