@Chris > Before you even scrolled down the preferences I could tell your players were not linked because the player number on the lower left of each display was grey instead of white.
My recommendation is that you try a different ethernet cable and check the jacks on each unit to see that there are no bent or misaligned pins. It could also be that one of the units has a faulty jack, the best way to troubleshoot that is to connect each unit individually to rekordbox on your computer directly, whichever doesn't communicate is the one with the problem.