Win7 computers don't see XP instance of networx

I installed networx last week and was using it to monitor my two Win7 machines.
Today I added networx to an XP machine as well.
Networx on the XP machine sees the usage from all 3 computers on the network but the 2 Win7 machines only see each other (the original set of machines on which the program was installed).
All 3 have the same settings - synchonise across all is selected as is ignore LAN traffic.
How can I resolve this? Currently the only "complete" usage is on the XP machine but its not a machine I use very often!

Thanks

Check or disable firewalls on these machines.

The machines see each other in every other way - I can transfer files between them - so I don't see how the firewalls can be a problem?

That's how firewalls work: allow specific activities and disallow others.

For NetWorx both incoming and outgoing connections must be allowed via TCP/UDP port 8703.

I use Comodo Firewall. I had already set Netwox as a "trusted application" on all machines...