Assuming it is programmed correctly, HP2 technology is "older" and seems to require alot more signal strength on a sample 700mhz P25 system. With the same antenna (stationary); my SDS200 reception is full bars, 536HP is 2-3, HP2 is 1 bar sometimes none. Doesn't mean its a bad unit, I love both of mine; it just is surpassed with newer technology.
Take the HP2 closer to the site(s) you are trying to monitor and hold on the control channel. You should be able to tell rather quickly if it is signal strength or programming.
Both can be improved, just need a proper direction to go.