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One of the biggest challenges in understanding and applying agile is often not the replication of the methods but the adoption of the mindset necessary to make them successful.
Without a predictive, long-term plan (as in waterfall), we can't know when/if the requested scope will be delivered. That's a lot of uncertainty. Using agile, however, when predictions are made, they are more accurate than if made up front. Also, the customer (via the Product Owner) has the authority (and responsibility) to provide frequent, regular input on priorities.
Customers must be engaged and stay engaged (via the Product Owner) in order for the process to be successful.
Project managers used to put a lot of time and effort into constructing a waterfall schedule. It is understandably frustrating when the work or the contributors themselves deviate from the plan. Project managers can remind contributors that, in agile, everyone worked together to create the plan; can (and should) ask the team if the plan is working; and, if it's not, coach the team to offer solutions. Remember that the goal is to make the team effective and accountable, and not to focus on any one individual.
Resources
Video [free]: "Hitler at a Sprint Review" - Meant to be humorous, based on a common meme. Please pardon a bit of colorful language.