I have the oddest issue. I can get my local PD all day long. When the sun goes down I get nothing from them. I live two miles from the repeater. I can get agencies on another network that uses the same repeater.They are both P25. I know it’s not a signal or interference issue. I’m at a loss. I know my scanner is older but it should be all or nothing.
Lots of ideas as I run a PSR-500 all the time. First thought: if this is a trunked system, does their control channel change at that time? Have you tried to park your scanner on that frequency in NFM mode? If it is conventional P25, what changes? (including NAC code). If it's trunked, just park the radio on the cc and see what happens to signal strength and decode rate.
What kind of antenna are you using? I know the desired tower is close, and it will make you think you are OK with an indoor antenna. But for me at least, the indoor antenna picked too much other stuff like PCs and electrical devices. Once I got the antenna outside and away from any potential interference, my reception (quality) went way up.
The PSR-500 has some settings in regard to how it handles a list of control channel frequencies. One gives you what it thinks is the "best" one which may not always be the case. There's also a threshold setting for when it will seek or switch to another. Just ideas.