We have that problem all the time, depending on how strong the channel signal is and what the weather is doing.
Also, we have a fan wired on the same circuit as one of our tvs. If the signal is weak and the fan is turned on, the signal goes away.
Sometimes, our two tvs on the same antenna pick up different channels (one will get 10 channels, the other will get 15). Sometimes a channel will go out on one tv but not the other. This is due to the tv's internal electronics, not the antenna.
eta: our Samsung picks up more channels than our Sanyo.