The GitHub integration for Slack is great for team productivity. Having the ability to give Slack channels full visibility into GitHub projects, for real-time notifications, is an amazing way to keep the development team on the same page. However, this can become a double-edged sword. Developers may decide to ignore these messages because it may be information overload.
I’ve been in a similar situation where pull requests needed approval from reviewers but they wouldn’t check the channels or their emails. The only thing that got their attention were direct messages. This hindered delivery and having to ping the reviewer myself was becoming a pain.