Worked Fine For A Few Days

I live in the middle of Missouri about an hour from the NBC/CBS stations. I have a digital TV with a rooftop antenna. When we first plugged the TV in last year it worked great. We picked up digital channels 3 hours away. The next day we got nothing. We've been watching the analog channels since.
When the switch happened last week we picked up 3 new channels. It worked fine but at night we would loose our signal. It worked fine for 4 days but for the past 3 days we haven't had any digital stations. I've been turning the antenna/ rescanning over and over and nothing comes up.
I don't understand why it worked fine for a few days and nothing comes in now.
We have a Samsung digital TV and I'm not sure if I'm scanning under the right setting maybe? You can have it scan under Digital STD, HRC or IRC....does that matter?
Thanks in Advance!