Scrum Teams are self-organizing and cross-functional. Self-organizing teams choose how best to  accomplish their work, rather than being directed by others outside the team. Cross-functional  teams have all competencies needed to accomplish the work without depending on others not  part of the team. The team model in Scrum is designed to optimize flexibility, creativity, and  productivity.

That's a big ask. 

Retrospectives ask us to focus on people, their interactions and the impediments between them. Each individual in the team brings skills and experience in a range of fields, a personality, values and motivations. They also bring baggage, pet dislikes, idiosyncrasies as well as skill and experience gaps needed to deliver working software.

All too often, we as Scrum Masters do not have the capability to hire and/or select to build teams from scratch. We have to work with an existing team.

We start working with the team and record the emergent interactions. Inevitably, we'll find that the team composition is not optimal.

How can we tell if a team is not optimal? 

As with any other impediment, this will show itself via a number of symptoms:

• A "crunch point" in some crucial part of the delivery mix - stories constantly not having anyone capable of designing UI for them, user story delivery hitting roadblocks at the same stage.
• A "crunch person"  someone, who despite attempts to cross skill within the team, may be an irreplaceable skill or expertise not currently found in the team.
• Specific individuals within the team just do not gosh-darn-get-along. 
• The team thinks of people in the team as "business analysts" and "testers" in specific roles, as refuses to complete the skills of those roles. Remember that scrum recognises no role other than Developer for members of the team that are not also a Scrum Master or Product Owner. This should be addressed via the introduction of cross-skilling. Encourage team members to go beyond their job role to expand to help the team where need may dictate and interest or experience might align with.

How might we address it? 

• Within the team: if it hasn't already been tried, try to cross skill. Focus the team on "what is needed to deliver this story completely" and flesh out the competencies required to achieve this.
• Outside the team: Look further afield for the required skill or expertise. This is not empirical scrum advice (ready your tomatoes for throwing), but swapping team members between scrum teams (if at a scaled level) and/or hiring new people with the correct competencies can help remove this impediment. Treat this as an empirical experiment. Add the competencies you think are required, inspect it for a few sprints, adjust where required.
• Scrum does not change personalities; and thus if an individual or a group of individuals is not the right fit for the team, it should be raised as an impediment for people moves between teams, or further escalation via management or HR. Often everybody is better off having had made this visible and grateful for having action taken on it.

So what's to be done? 

In my experiences so far, team composition is one of the most crucial components to the success or failure of a scrum team that ensuring you have a mix right is a priority. 

Engage with your executive stakeholders early on and lay the groundwork of their expectations to give you the ability to make recommendations (and better yet, direct decisions) to influence the composition of the scrum teams. 

Then use the techniques described above to identify and help to optimise the teams accordingly. Let me know your thoughts; or how you go trying this out!