Weird blocking of .NET (only!) Internet traffic

We have a POS-based application in Windows Forms written in VB.NET, which we're rolling out to franchise stores - 70, so far.

The application replicates data by means of a connection to a head office server: all communication is by means of Web Services (upload & download).

All is working really well, except - you knew it was coming <g> - on literally a handful of stores we cannot connect to the web services, although we know that the Internet service is good!

Here's a bullet-point summary of the situation:

1. in this handful of stores, ALL .NET (only) internet traffic fails on this POS computer.
2. primarily this traffic is using port 80 to connect to web services, using SOAP (standard)
3. BUT they can successfully use a browser in the POS to see web pages
4. and a test program I created in Clarion (straight Win 32) also connects to the web service ok.
5. In addition, an associated data conversion app (again in VB.NET) also fails when trying to connect to MS SQL at HQ (and only for this handful of stores).
   Note: this uses a standard ADO connection, thus a completely different protocol to the above.
   
6. So it's just all .NET Internet that fails... different apps, different protocol.
7. The error message in all cases is "The operation has timed out"
   
8. Note: timeout is set to 15 seconds - ample time for simple web service, esp. as the initial method is just a time-check.
9. Take a POS that's failing to connect and move to a different site: all good
10. Take a POS that's failing to connect and change the router: all good
11. But the routers are *not* all the same

It looks as if the .NET setup on the POS pc is somehow being 'blocked'.

Too weird - what am I missing here?

Grateful for pointers... I'm stumped!
Steve