@tibblet 

Hi,

Distance and interference are the main factors that affect the performance of these Powerline adapters.

Obviously, if you can't move the Powerline adapter that is connected to the modem, then you can only try to move the other Powerline adapters closer to the one that is connected to the modem.

Regarding the interference. You could try to find out whether there is one particular electric appliance nearby that causes a lot of interference. 
To do that run the "tpPLC Utility" software on a computer and observe the "Powerline rate" between the TP-PA7017P units. (note that the "Powerline rate" doesn't indicate actual data throughput speed)
Let the values of the "Powerline rate" refresh a few times to kind of see where the mean values are.
Then unplug electric appliances that are plugged into sockets along the way from one Powerline adapter to another one by one and every time check whether the "Powerline rate" increases significantly after you've unplugged something. (don't forget to unplug the power adapter of the modem and other "small" things, as size doesn't matter here)