I drive up & down US13 every week; never have trouble. I would say one thing to check is to see if you can receive the data channel from Accomac, Eastville or both. You don't say where in Accomack County you're located. Possible you're in a poor reception area. Could be your HP1 is in the wrong part of the house. Digital signals don't need much to screw it up for us scannists. That's the difference between a $400 scanner and a $2000 transceiver (a tighter front end for the transceiver dedicated for a small portion of the spectrum vs a wide band 25-1300 MHZ receiver. Another possibility could be your scanner is seeing the signal but if it's also receiving a reflection from say, off a water tower (for example), that would be enough to make your scanner ignore the transmissions.
Not being in this area of Virginia, but I listen to STARS in NoVa in Division 7 a few times a week when there with the work truck and P25 Trunking works much better on VHF than 700/800 Mhz, as we have in Maryland. The sites all come in very clear and would think it's a programming issue. Have been as far a Hampton Roads a few times, since STARS has been in service and it seams to be a very well designed system.
The Accomack site covers very well by you - problem I have when traveling to VB along Rt13 is how can you tell which units are for the Accomack/Northampton; the site is busy with Chesapeake and Virginia Beach RT 64 area units - and I notice while units out of Melfa don't get many calls, most are past over the MDT/computer; at times seems there are only 4 units on average covering the two counties - a few more when the scales are open....
In the STARS Wiki there are some Eastern Shore Unit ID's, some are dated, but it works for me. The midnight roll call is also a good time to get the Radio IDs or unit numbers, if the trooper doesn't key up, the dispatcher has to pass there location out over the air.