According to the RR Database, the Philly P-25 system the PD are on is a simulcast system. There are known issues with (and various solutions to) listening to a simulcast system, most boil down to the issue of Simulcast Distortion. A search on these forums will provide you with many threads on this issue and there's even an article in the RR Wiki on the subject (
Simulcast digital distortion - The RadioReference Wiki).
Basically a simulcast system broadcasts the same signals on the same frequencies from various towers to improve coverage (especially for hand-held radios) in the prescribed coverage area. The expensive radios have special circuits to handle the multiple signals, while scanners do not. The multiple signals reaching your radio from the various towers cause multi-path signals (similar to the old TVs scrolling, flashing, or giving multiple images when a low flying plane flew over) giving you multipath distortion or what's often called simulcast distortion.
While it may look like you're having too little signal to properly decode the signal, most often the actual issue is too much signal reaching your scanner from too many towers at once. If you can configure your station so you only pick up the signal from a single tower, you'll generally eliminate the issues.
Things to try:
1. Turn on your attenuator (will reduce all signals and hopefully only the closest tower will be strong enough to decode on your radio).
2. Change to a very directional antenna (a yagi works for many folks) and point it at a tower that is isolated (this isn't always the closest, a nearby tower directly to your east with another one further east may not isolate the signals enough while a further away tower to your north that has no other towers in that direction may allow you to isolate the signal quite well).
3. Do something else that reduces the multipath signals (perhaps a mag-mount on your AC compressor vs. that $300 antenna on your tower) so only a single tower is received.
To help show you how weird solutions may work I'll tell you about how I'm able to pick up a local simulcast system on my Pro-197. I'm about 3 miles away from a tower for a local P-25 simulcast system (tower's to my north-northeast). There's another tower about 6 miles away to my east-southeast. Using my normal outside antenna I pick up very little on that system (generally just bits of a signal and some digital noise).
I was doing some cable management by rerouting some of the mass of coax in my monitoring position and when I powered my scanners back up I noticed that this system was all of a sudden fully receivable. I figured that I may have had a loose connection for that scanner (although this was checked and not found when diagnosing the reception issues) and gave it no further thought, but was happy that I could now listen to that system. Later on I added another scanner and when connecting it up to my splitter I noticed a coax that wasn't connected to anything. I traced this unconnected cable back to my now working scanner on the simulcast system. Turns out that the fix for this simulcast system for me was surprisingly to have no antenna connected to the scanner!
