I have a program that prints labels at five remote sites. Two sites, aren't working, but the rest are with an identical (as far as I can tell) setup.

Using Wireshark, I have determined that the handshaking all goes well, but after the "Open Print File Response" the packet that is sent from the server, doesn't reach the client. But I'm a bit at a loss as to where I go from here.

I know the port the packet was sent on (445) isn't being blocked, the RST packet gets sent on the same port and that gets there fine.

It's also weird that the three out of five sites are working fine.

This has been up and running for years without issue, all that we have changed is our connectivity (from DSL to bonded DSL). But this traffic is over a VPN - so it can't be the ISP interfering either can it?

I'm totally stuck, and any help would be much appreciated. Thanks!