Do your Sprint Planning Meetings last longer than 2 hours? Are there heated discussions about unclear requirements and does the Product Owner have a hard time to answer the questions? Is it normal that new detail requirements pop up during the Sprint you implement the according Product Backlog item? Do the developers just develop during a Sprint? Does the Product Owner gather all requirements and the developers do not have contact with users outside the Sprint Review?
If you answer these questions with “Yes”, you are probably not refining the Product Backlog and detail requirements for the next items in the Product Backlog prior to the Sprint Planning Meeting.