There is sufficient signal at your house for NBC and PBS. What's different about those two channels is that they are on VHF. The others are UHF. You have the right antenna for VHF.
So I would guess that the problem is caused by interference or something in your system that passes UHF but not VHF.
Typical sources of interference are:
LED lightbulbs
switch mode power supplies
Solar panel inverters, etc.
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