Well I did some further tests on the two bad pagers I have, and the results are interesting. It appears a component related to wideband (5.0 kHz deviation) reception has failed in these units.
When programmed to 154.600 wideband, in monitor mode, and using a PR400 on low power; the pagers will not open squelch at all with the radio just a few feet away. If I manually open squelch by holding down the reset button while transmitting, I can hear audio but it is so far down in the noise (and slightly distorted) that I can barely make out myself tapping on the microphone. If I transmit the tones programmed into the pager it still manages to alert as normal. I also noticed that when manually opening squelch, the white noise sounds low and muffled.
Using the same setup and simply un-checking "Wide band" in the pager software (and changing to narrowband on the PR400), the pagers receive normally. I tested this on both MURS channels that allow 5.0 kHz deviation and also an old fireground channel, and each time narrowband worked fine whereas the pagers were almost deaf on wideband. I also programmed the pagers to monitor the neighboring county's narrowband system (much busier than ours) with repeaters between 12 and 18 miles away. With one pager programmed narrowband and the other wide, the results were the same as when I tested them.
Anyone more familiar with the actual hardware design have any idea what would cause this?
Also, to the OP: What date code do your 'bad' pagers have? It is made up of the second and third letters in the serial number. Going through our files, it looks like every single one of our 12 "JC" dated pagers have had this same problem.