Two things to try:
1 - Go to the FCC Licenses tab on each county's database page. There may be more frequencies licensed, than what you see identified in the database.
2 - Try setting your squelch type to 'search'
If you read about my adventures in southside Virginia the last two days, you see that some locations I thought I knew well changed on me over the last year. In some cases they changed frequency. In other cases they just changed the squelch code. Coded squelch really combats the interference, but if you put in one code, and the user changes to another, you're going to have a silent frequency.
Let us know what you find.
73/Allen (N4JRI)