Why did the automatic alignment process fail?
Scans with little distinguishing geometry, caused for example by a lack of features or large flat surfaces, may not provide enough data for the automatic alignment feature to work properly.
While scanning, try to ensure that there is plenty of overlapping and unique data. If you are moving the Kinect around an object, try not to do it too quickly.
In some cases, such as when scanning a cylinder, it may be necessary to place other objects in the scene, which act as "anchor points" during the alignment phase. Once the object has been completely aligned and combined, the extra scan data can be deleted.