The checklist item that is the most likely reason why the stories are incomplete at the close of the sprint is small. Small means that a user story should be simple and concise, and it should be able to be completed within a single sprint by a single developer or a small team. A user story that is too large or complex may be difficult to estimate, prioritize, test, or deliver within the sprint timeframe. A user story that is too small may be trivial, redundant, or irrelevant to the project goals. A user story that is not small enough may need to be broken down into smaller and more manageable chunks. References:
https://trailhead.salesforce.com/content/learn/modules/user-story-creation/write-user-stories