Could be 2 things:

Check that you are not operating the PM/Catalyst server on the same artnet universe as you are trying to output to the pixels...
Also check the Mac IP adress is not the same as any artnet interface and in the correct range. Turn off the 2nd network port if not in use.

However, it sounds like its due to multicast...

How many DMX universes are you running?

Try setting each PM DMX output ip address (on the universe map tab) to the matching artnet interface adress for each DMX output subnet.

This means that the information for each DMX universe is directed to a specific artnet i/f box - not multicast everywhere...

Don't run anything else on the same network (ie content uploads etc) - ensure its just the Catalyst/PM server and the artnet/dmx boxes.

Can you post a screen grab of your Artnet window, showing both 'interface' and 'universe map' tabs

Simon