The problem is the HP units are not sophisticated enough to easily handle P25 and simulcast systems. The HP units are also susceptible to interference from cell towers, other radio transmitters, etc. (I use one in the Phoenix area and it requires a fair amount of manipulation instead of just entering your zip code.) It appears Liberty PD is not encrypted so you should be able to hear them. It's also not likely an antenna issue if you're in Liberty, but it may be something to consider if all else fails.
It's best to start simple and work out from there. I'm assuming you know the operations of the HP2 beyond just entering a zip code. If not and you need more specific instructions, just let me know.
1) Using the Radio Reference database for the MARRS system identify which RFSS site is closest to you and likely to carry Liberty traffic.
2) Lock out every other site.
3) Identify the control channel frequency for that site and lock out all the other frequencies.
4) Stop scanning and "hold" on one main Liberty channel (either police or fire.)
5) Leave everything alone now and just listen for a reasonable amount of time.
6) If you still hear nothing, try holding on another Liberty channel and listen.
7) If you still hear nothing, try repeating these steps using the alternate control channel.
8) If you still hear nothing, re-enter the original control channel but try another nearby RFSS site.
As you can see, it may require a lot of trial and error. Another option, depending upon your patience and technical expertise level, you may want to consider returning the HP2 to the vendor and upgrade to the SDS series. The SDS will eliminate all of this hassle.
