PLP2000 using more than 4 units to avoid using wifi

 I have four units connected perfectly on 3 different floors of the house, some plugged into new grounded wiring and one plugged into old ungrounded wiring, but every time I try to add a fifth unit, it doesn’t seem to be able to find the network.  Documentation says that you can add up to 16 adapters to extended your network. Any suggestions on how to make that happen or what are the blockades that prevent connection.