Three possible reasons:
1) Most likely it is the distance to the transmitter and weather conditions. You might have scanned on a bright starry winter night with ideal conditions. Some channel strength has dropped below the threshold on the next rainy windy day. You would only reliably get channels within 20 miles on a bad night and up to 30 miles ideally on a good night with your attic antenna.
2) It is the ongoing FCC transition. You have to re-scan every so often in the next couple of years.
3) Please check coaxial cable connections. I recently "lost" half of my 58 channels only because I shifted my TV and touched the antenna cable.
1. From the home screen, select Settings.
2. Select TV inputs.
3. Select Antenna TV.
4. Select Favorite channels
5. Press OK to uncheck Enable ‘Favorite channels’.
b. Select TV inputs.
c. Select Antenna TV.
d. Select Favorite channels
e. Press OK to uncheck Enable ‘Favorite channels’.
f. Go back to Antenna TV and see if you have access to your channels again.
2. Drain the power of the TV.
a. Turn the TV off and unplug it from the outlet.
b. Press and hold the TV Power button on the bottom surface of the TV for 15 seconds.
c. Turn the TV on and try again
3. Connect a different antenna to the TV.
4. If the TV is connected to the TV, disconnect the TV to the internet and scan for channels again.
a. From the home screen, select Settings
b. Select System
c. Press OK on Advanced system settings
d. Select Network connection reset
e. Press OK on Reset connection
5. Perform a factory reset. Please take note that this resets the TV to its factory default and clears the Roku account in the TV.
a. From the Home screen, go to Settings
b. Select System
c. Select Advanced System Settings
d. Select Factory Reset
e. Select Factory Reset Everything.
Yes this happens and its annoying. I dont believe its Brand specific. Some people report Great Success with wrapping your antenna with foil. Obviously you need to adjust your Antenna for some channels.