It's well-documented on the forums that these scanners (and the newer GRE line of digital radios) struggle with P25 simulcast systems like that in Chas County. Its the LSM modulation used in the simulcast transmissions that seem to be a problem for scanners. I had the same problem with my PRO-96 on the Charleston County system and others have reported similar experience. The PRO-106/PSR-500 is better but still not great. Search the forums and you'll read similar reports.
The McClellanville and Mt. Pleasant sites aren't simulcast, I don't think. But you won't hear ALL traffic on them either and their coverage is limited.
The common suggestion is to use a directional antenna that will help isolate reception to one (the nearest) tower. That's impossible when using the radio in a portable or mobile fashion.