By design, the radio will not do what you are wanting it to do.
A public safety radio is not meant to be used as a scan-all-the-things radio. It is meant to sit on a channel and wait for a call, and scan a FEW channels in the background in the event someone calls on a secondary channel. Also, the radio will only hear the calls on the site it is monitoring. It does not care what other sites are transmitting, it will drag the channel you are sitting on to the site you are on if needed (if login/affiliation is enabled). Otherwise, in this "passive" mode, the radio is at the mercy of whatever the site happens to be transmitting. There could be a hot call on the town over with lots of chatter but your site doesn't have any radios affiliated to that group, so none of it makes it there. The radio will not look for the group elsewhere, its priority is to find the site with the best signal (these are designed with life-preservation in mind, i.e. needs to be able to get out all the time), and just bring the group with it. Popular channels that are regularly scanned by end users can be forced to all the sites in a region (e.g. a car-car or "chit chat" channel). That way, a user that primarily sits on the main dispatch channel will still hear the supervisor call for them without the need for dispatch to tell them to switch over.
Your best bet is to either A. accept that hearing everything everywhere is just not possible. or B. compromise and set the radio up to be site-selectable, that way you can at least lock the radio onto the site with the most traffic instead of letting it roam where it wants.