Two AEpress 2013 extending a wireless network created in a AExtreme

I'm trying to extend wirelessly a network created in my AirPort Extreme 2013, using a couple of AExpress.

The configurations are perfects, I can see everything correctly from Airport Utility, but only one of the two AExpress is accepting clients connections.

So I have the AirPort Extreme with 6 clients connected (Apple TV, iPad, etc), the 1st AExpress with 4 clients connected (Mac, iPad,, etc), and the 2nd AExpress with no clients connected (also when I try to connect to the same SSID from a clients that is located just some feet behind the 2nd AExpress).

How it's possible? How can really extend the coverage range of my network? (Both 2.4 and 5 Ghz?).

If only the 1st AExpress is sufficient, why in the room where is located the 2nd one I loose the connection if the signal of the 1st one is too poor and the device doesn't try to connect to the 2nd and nearest?????