Band conditions change... As the band "Fades" out you loose contact with the staiton, A more powerful antenna (Winegard Sensar IV with Sensar Pro for example) might be able to hold it.. Or not depending on how deeply it fades.
At UHF frequencies even the sun affects the signal strength. As do sun spots and many other things.
I have sat here in my motor home listening to people talking on High Frequency (A thousand times the wavelength) and heard them go from "Next door" quality to "Where'd they go" in less than an hour. UHF is not usually that bad, but it can be.