Hello,
My team is working on complex project, and our Product Owner has created a very clear PBIs at the beginning, and team starts to forecast these items into an increment, but lately the complexity of project is raising and every item added to Product Backlog is complex, and now the team is complaining since new PBIs requires more time than what they have progressed before. But product owner and stakeholders they are pushing the team to be more productive so the goal of product can be done on the deadlines.
As Scrum Master, I recommend