Check out this article and linked FCC PDF:
http://www.hometheatermag.com/news/061809dtvweak/
How to "Double Rescan" a DTV:
1.Disconnect the antenna from the box or digital TV.
2.Re-scan the box or digital TV without the antenna connected. As with any scan follow the on-screen instructions or owner's manual for your device.
3.Unplug the box or digital TV from the electrical outlet for at least one minute.
4.Reconnect the antenna to the box or digital TV and plug the unit into the electrical outlet.
5.Rescan the box or digital TV one more time.
Here are the FCC's tips on indoor antenna location:
Near a window.
As high as possible.
Away from other electronic equipment, including computers, VCRs, DVD players, converter boxes, and the television itself.
Change the direction the antenna is facing.
Rooftop antennas may be needed in some instances.
And don't forget to run the channel scan again after moving the antenna.