I am trying to set up 2 extenders to my router's so that it will reach two dead areas in my home?

I am trying to set up 2 Netgear EX6150 wifi extenders on my router BT Home Hub 5, as I have an old house with thick solid walls and plenty of  deadzones.  My router is on the groundfloor on an outside wall at the front of the house.  I have installed power lines with a repeater (TPLink) at the rear of the house which works fine.  However inbetween, there is a deadzone, and upstairs it is very patchy.

So my idea was to bounce the signal from the BT Home Hub into the mid point on the ground floor (my kitchen), and so was initially happy with my purchase as when I have plugged in one of the 2 new Netgear extenders, having made the connection via WPS, I can get a good link in the kitchen.  However, when I try and get a connection via WPS and plug in the second extender it won't connect at all.

Is there a way round this seeming conflict?

Any help or suggestions would be most welcome... as it is the summer holidays and as you can imagine wifi for the kids is pretty vital and without it working well they are desperate!  ... As am I!

Looking forward to your views