What's your effective collaboration?

In our previous experience working with external design agencies, we've sometimes encountered frustrating situations where communication broke down. For instance, we’d send feedback on a design, only to find that it hadn't been properly communicated to the design team, leading to wasted time and duplicated effort. So, to avoid repeating these issues, we'd like to understand: What strategies do you use to facilitate effective collaboration and ensure that everyone is on the same page?