What is the one thing that you can focus on fixing in an Agile team to start your journey towards an high performing team?
Fix the attitude of "My way or the Highway". Identify who has this attitude in the team and fix that first. This includes Scrum Master as well.
Encourage an environment where collaboration becomes a culture. Every thing with respect to product must be discussed as a team; get this into the team agreement. Things will start to take shape in the right direction.