The Bottleneck issue

Are you terrified of going on vacation? Can you already guess all the disasters that will happen when you're out of office for more than 2 days?
If yes, you're not alone, but there is something you should do about it.

Oftentimes, new Agile teams have the "Problem" that the intensity of work is so high that people are somehow stuck in their role.

For instance, the Scrum Master may not even get to coach the team because impediments and ceremonies already fill their schedule: The installation of the Continuous Improvment Process may become secondary.
Or, the Product Owner is spending fulltime answering questions of team members to ensure the product is terrific: Good, but the grooming of future backlog items and stakeholder management may suffer.
The worst case is developers taking so many stories that they lose time to hone their agility: The delivered product may be great, but we didn't implement any ways to do the same thing faster and easier.

The Solution

Agile methodologies try to eliminate "Single Points of Failure", i.e. having success or failure hinge on one single person.

• Developers should ideally pair up to make sure that there is no capability required in the project which only one person has. Why? Isn't it more easy to replace you if some else possesses your skills? No! On the contrary: if you can chip in wherever something needs to be done, your value to the company increases!
• The Product Owner, "owns the product" and needs to call shots. However, they should not feel obliged to create a structure where every minute decision entails them.On the contrary, the PO should communicate the vision so clearly that the team can independently decide what is the best way to advance the product. 
• The Scrum master, "owns the process" and is responsible for ceremonies and managing the impediment backlog. A good Scrum Master will not spend their days running after individual impediments and arranging meetings. Ideally, they will coach and empower the team to do this by themselves. 

For both the Scrum Master and the Product Owner, I would refer to an important proverb about aid: "The most important job of a helper is to make themselves superfluous", with the intention "If you enable and empower others to fill your role, you did a good job, otherwise you missed the mark".

Sustainability is one of the Agile Principles, and you will not have sustainable development unless everyone actually strives to enable others to do what they are doing.