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. Re-scan the box or digital TV one more time.
I got this suggestion from my local PBS station when their three broadcast stations mysteriously disappeared from my TV and a conventional re-scan did not retrieve them.