Are you sure the Sceptre has an ATSC digital tuner? Is it set to over the air (rather than QAM)?
The tuner in the Sceptre may just be bad (or lousy). You could try an outboard tuner. Some have PVR capability and even two tuners so it might be worth a try.
If you are limited to indoor antennas it can be hit or miss. You can check out Channel Master
http
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/www.channelmaster.com/