I had quite a bit of activity on 7777 the past few days. It was multiple users doing "radio checks" (on the audio clips). Maybe it's another agency about to move to the system. We know that Ellis County as well as Ennis plan to move, but so far I have not yet seen anything that I could pin to either of them.
I was looking at this system today. Earlier, I heard what was probably the dispatcher side of a conversation for Waxahachie PD, but I did not hear any sort of a reply.
I parked my SDS200 on the Waxahachie PD TGID for a while. Seeing a number of transmissions, but the scanner display is calling them "data". I have a suspicion that it may really be encrypted, but a format that the scanner thinks is data (not seeing the normal ENC show up). I parked the scanner over on the Midlothian PD dispatch TGID for a while, & saw the same thing happening. When Midlothian first moved to this system, as well as when Waxahachie first joined, I had no problems hearing either of them,
(As I was typing this, again heard the Waxahachie dispatcher as for a tag #, but did not hear a voice replay.)
If, as in your first post, you are hearing Fire & EMS on the trunked system, then it is not an antenna issue, and probably not simulcast issues either. I'm well out of simulcast zone issues, and the SDS series scanners handle simulcast really well anyway, then I think they may have decided to 'go dark' (encrypted). If that's the case, then no scanner can overcome encryption. I'm seeing much the same as you, so it's not a problem with your scanner.
(Just now, another long conversation from dispatch, but no voice reply from field units.)