User Stories are Half the Story
There are many ways in which you can capture requirements. The current "en vogue" method is to writ down user stories on index cards and stick them on a white board. A structured approach to requirements is key if you want to ever finish your project, unfortunately user stories fail to capture all aspects that define a piece of software.
The first and obvious contender to missed requirements are the non functional requirements. Non functional requirements are the ephemeral requirements that encompass maintainability usability, performance and memory footprint. Many methodologies try to skirt the issue by defining "quality metrics" or "common values". The idea here is, that by creating the cleanest and optimal code you actually implement the non functional requirements because the code is is such an awesome state. The problem is that the optimal solution solution for ten elements is not the optimal solution for one million elements.
One of the aspects encapsulated in properly defined non functional requirements are the performance characteristics with a certain input. Unfortunately this does not fit onto an index card written in the manner "As a USER, I want GOAL as to REASON". You can write "As a user, I want to wait no longer than 1 minute on my results, as to not waste my precious time." But what is the completion criterion.
You need to define you non functional requirements, you can call them quality metrics if you like.
The second and less obvious contender to missed requirements are the collateral features. The features that everybody expects a good piece of software has, but can not be pinned to one single task. These are features like, undo, copy and paste. Optimally you want to undo each input and copy each element. It is conceivable that one common value, in the context of usability, is that each action should be undoable. The problem here is that it creates a constant drag on development and it is not a far fetch that undo provides less business value than the next proper feature. If the later is the case, basically each collateral feature needs to be captured through a user story and scheduled. This results in a large number of little user stories, but it is the better solution.
So next time you sit down to capture requirements into user stories, don't forget to also capture the non functional and collateral requirements.