Extreme and Express extending issue

i have a Extreme 802.11n and a Express 802.11g,
I am trying to extend my network, so all my devices will get full signal anywhere in my house. i have set up my Etreme as create wireless network,
do not have the 5GHz checked, and i do have the EXTEND THIS NETWORK
checked, and i have the radio mode set to 802.11 a/n - 802.11b/g.
when i set up the Express i have it set to join wireless net work,and to extend
the network of my airport extreme,
when i'm done doing the set up, i get the BIG CHECKMARK, with the
congradulations, your network is now extended.
So i have the Extreme in the Southend of my house and the Express in the
north end of my house, when i'm in the southend i have 100% signal, but when
i move to the north end of my house the signal drops to 25%, it's like the Express isn't extending my network, i thought extending
my network i should have full signal at both ends of my house.