The analog signals were viewable as long as the channel strength was above the noise threshold. They might have been snowy and unwatchable, but you could brag! Digital channels require significantly better signal quality over a longer time to gather enough data to build an image or audio track.
Often I can read enough digital data to display the channel ID, but not adequate to produce picture or sound. Use TSReader with a Silicon Dust and see the tables and pids on channels that don't have adequate signal to display programming.
For the next 6 months we reliably watch San Francisco/San Jose channels in the early morning and primetime, while the local Sacramento channels disappear. Fun for me, but aggravating for my wife!