Every time you set up the TV sets at a new site, do you orient the antenna and then perform a channel scan? Many new sets with digital tuners need to scan the full spectrum to figure out what channels are where, and that can change if you move, since the local broadcasts may be on different frequencies.
This would also explain why one set will pick up channels that the older one doesn't. The older one may simply not know where to look, because it hasn't scanned the spectrum.
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