Help with OTA antenna

using channels for a couple of weeks, like it so far. not really a channels issue but thought someone here could offer some suggestions... My OTA antenna is in attic right at a window. All my network stations are like 20 miles away, seemingly right by each other. They all say around 100% signal on HD Homerun signal app. However, my local CBS (saying 100% singal, SNR varies 60´s to 70´s) will pixelate and for moments all the signal bars on the signal app will go grey and I will lose the channel briefly. I´m confused as all the other channels work great, all have green bars on the HD signal app, and all the towers appear to be right together. Also, from what I gather, 20 miles is nothing (hence the 100% signal?). This is the only channel I ever the see the SNR go to yellow so maybe that has something to do with it? I lose the channel altogether (though briefly) so I guess it´s something other than signal strength. Any tips? thanks.

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