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.