To the OP, I saw where you mentioned that you are only 5 miles from the P25 site you want to monitor. Program that trunk system into the HP1 scanner by itself. If the site is simulcast, try listening with your attenuator ON. Sometimes being close to a simulcast system will make the multiple signals tough to decode.With the attenuator on it will not hear the weak towers and will lock on to the nearest towers. If you have an outdoor antenna with directivity such as a yagi, play with that as well. every system decodes different. One simulcast system near me I need a long Yagi right on the closest tower and it decodes fine. Massive S-meter reading on the HP1, but decodes great. Another simulcast P25 system about the same distance overloads on a strong signal and will not decode. Have to run it with the attenuator AND turn the antenna away! Only decodes with a tiny signal. When I look at this system with the trunk analyzer the bar graph shows practically no signal and 95% quality. Craziest thing I have ever seen. A preamp can also be in the mix in combo with the attenuator to get the best decode. Once you are comfortable with the decode you can program a favorite with that and other systems.
I will give an example of the craziness. One P25 simulcast system works great with the 900 yagi, my pre amp, and a cheap bnc to bnc jumper to a uhf to sma adapter on the HP1. Decodes every call clear as a bell. Now I changed that cheap jumper out with a short piece of LMR400 with UHF connectors on both ends and a BNC adapter going to the preamp. The other end into the UHF/sma adapter on the HP1. The system disappeared. I experimented with the attenuator, made sure the LMR400 jumper was ok, could not get it to work. Put the cheap low grade BNC-BNC jumper on and it worked again! So with the loss on the cheap cable the signal would decode. But with the good LMR400 there was less loss and the signal would not decode. Hope that helps you out some. I have an HP1, 996XT, and a 396XT. They all have the latest firmware. The HP1 is the most finicky on decoding digital, but in my opinion it sounds the best with hardly any chop or errors.