That's the nature of a networked trunked radio system. Talkgroups are generally only heard on sites that have subscriber radios affiliated with that talkgroup. It's likely that whatever detail you're listening to has radios mostly subscribing to the Florence site and not often to the Darlington site, based on the location of their operation. Hence, you're not hearing the channel on the Darlington site continuously.
If you're so motivated, using one of the control channel decoding software applications, such as Pro96Com, or Unitrunker, or DSD+, will show affiliations, de-affiliations, and denied affiliation attempts on a particular site. That might be able to provide some clarity as to what exactly is happening. Without that, we can only speculate.
I guess it's just the number of radios from the one area that they are originally affiliated with such as a home channel I used to be with palmetto rural and the 800 walkies always stayed with the Darlington site