The image below shows a poor scan on the left. Scanahand failed to detect the template because the horizontal and vertical grid lines are interrupted. On the right you see an example of how a scanned in template should look like.
So if this sounds familiar, do make sure your printer is working the way it's supposed to be. So don't print with low toner or low cartridges. Also make sure your scanner produces high quality scans. Consult the documentation that came with your printer and/or scanner for more information.