The biggest problem I've faced is team culture. It's impossible to adopt an agile solution for a team that thinks "waterfall." Even if you introduce agile practices into your team, if the team still thinks in "waterfall" mode, then people will naturally gravitate back to what they know and what they're comfortable with.
My team started the project using agile, but as soon as my workload increased and I wasn't able to "police" the processes, everything got messed up. I suppose this means that I didn't infect other people enough to get agile a self running process.
If you have a medium sized team (about 20 people), how do you keep the team on track with agile and not "fall off the wagon" into bad practices?
The only way I know of doing that is by involving them in shaping the process.
Do you do iteration retrospectives?
The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus
Although only covered lightly in the book - the chapter on Personal Agility - the idea of individual responsibility knowing "what's in it for me?" for each team member is a foundation of any successful, high performance team. Understanding the human relations foundation and team makeup was out of the scope of this book, but I believe that is what you are observing.
I am the new editor for the Agile Journal and we'll be covering the human dynamics regularly from now on. A good starting point for understanding these team dynamics is the
Amr Elssamadisy<br /><a href="http://www.amazon.com/Agile-Adoption-Patterns-Roadmap-Organizational/dp/0321514521/ref=pd_bbs_sr_1?ie=UTF8&s=books&qid=1220909336&sr=8-1" target="_blank" rel="nofollow">Agile Adoption Patterns</a>