Networks require a Common Ground
When systems in the same room are networked, it
is usually easy, but most the time,networking connects systems
over a much longer distance, and then an additional factor pops
up: the "ground-level" of the power-line.
I have been confronted with situations, where there was a
substantial difference between the ground-level of systems,
because they were connected to a difference phase of the
power-line or even on power-line segment, supplied from different
transformers. If this difference of the ground-level gets too
big, the systems cannot anymore read the Ethernet signal
properly, on some installations the network become
"slow" (caused by re-transmissions) or the
communications breaks down completely.
In commercial installations, special converter box (or even
"fiber glas ", segments are then installed to 'uncouple' the
signal from the ground-level of the systems).
If you suspect in Home-installations such a problem, install
temporary the systems next to each other in the SAME room,
getting the power from the SAME power-outlet, but STILL use
the same length of Ethernet-cable. If the problem now disappears,
then it is likely such a ground-level problem.