Most likely the situation is , signal reflections = multipath.
Omnidirectional antennas do not reject multipath signal reflections.
Try other locations with the omnidirectional antenna.
If the situation is not resolved.
______________________
Install a Directional antenna.
Install a Winegard HD7696P antenna with no antenna amplifier aimed at about 219 degree magnetic compass direction.
WJFW Real UHF channel 16 , virtual number (12.1) NBC.
And
Repeater station W27AU-D Real UHF channel 27 , virtual number (12.1) NBC.
Are the same Tv station.
http://en.wikipedia.org/wiki/WJFW-TV.
So that puts all the Tv stations/channels in one direction.
_________________________
As always trees and tree leaves do a real fine job of , reducing , reflecting , blocking , Tv transmissions.
Locate the HD7696P antenna where there is the least amount or no amount of trees in the direction of south west the direction of reception , about 219 degree magnetic compass direction.
___________________
Always rescan for channels after moving a antenna or installing a new antenna.
_____________
http://www.solidsignal.com ,
http://www.winegarddirect.com ,
http://www.amazon.com.