By chance did you have the input NMSP freqs turned on in the HP? Since NMSP doesn't use repeaters in the sense that we are used to, you have also listen to the input freqs on their repeaters to hear conversations. If you do a search on NMSP in the forums, there are several threads about this topic.
And it's at best if you get to hear both sides, such as a unit near by or in a decent area. Mostly one sided. Keep in mind district local uses diff then district district units. Eventually the conventional p25 system will be in place. For the most part it's ready to go.