According to the Project Management Professional (PMP) guidelines, when a team is struggling to complete work due to unclear requirements, it's often beneficial to review the backlog refinement processes. The product owner, who is responsible for managing the product backlog, should review and refine the backlog items regularly to ensure they are clear, actionable, and prioritized. This process includes detailing the requirements, ordering the items in the backlog to best achieve goals and missions, and optimizing the value of the work the development team performs. By doing so, the team can have a better understanding of the requirements, which can help them complete the iterations successfully. References: Agile Practice Guide, PMI, Chapter 4: Implementing Agile: Delivering in an Agile Environment, Section: Backlog Refinement.
Project Management Professional (PMP) Certification Handbook, PMBOK Guide 6th Edition, Chapter 5:
Project Scope Management.