Do you have the antenna aimed correctly and the correct antenna for the distance between your location and the stations? Check antennaweb.org and tvfool.com for the directions to the stations you should be able to receive. Antennaweb assumes an external antenna and is very conservative in the station list. TVfool gives the stations you should get with various different types of antennas. Tall trees and buildings will block signal strength so consider the environment around you.
With digital signals, if you do not have enough signal strength for a given tuner, the channel will not come in. An amplifier may help you get the signal, particularly if the signal is shared to several devices. Failure of an amplifier or lightning surge protection can stop the signal from reaching the TV. A surge through the antenna can damage the tuner.
A damaged cable can also be a source of a problem. Check the outside portion of the wire as well as the interior wires. Replace any that have damaged insulation or ends.
For this TV, if you have done a channel scan with the TV set to the Input TV/DTV, and the Signal Source set to Antenna. Turn the antenna and then set the Additional Scan to On. (Menu > TV > Channels > Additional Scan.) Then do another channel scan.
If you need a copy of the manual, see: http://www.vizio.com/m220va.html#support