While I can't diagnose your problem I will say that growing potatoes in a container has its own set of problems.
One, is that many growers dont assure the container efficiently drains away excess water, which can rot your potatoes. Secondly, potatoes really need lots of sun. Often times, especially as the container grows heavier with the continued addition of soil, the container isn't moved to a good, sunny location.
Lastly, when adding the additional soil throughout the growing season, it's important not to entirely cover up the green, growing top. It should be left out in the open to convert the sunlight in its growing process.
If you've taken care in the above areas, then perhaps someone else has a better idea as to the failure of the crop...
Best of luck!