Question Wifi Range Extender Signal Drops


Sep 4, 2009
Recently I purchased a Tp-link Range extender(TL-WA855RE). But I am having connection issues with it. I have a cable broadband(100 up,50 down) and My main router(Netgear WNDR3400) is in the ground floor. I'm getting poor wifi reception on first floor. So i bought and I put the extender in middle of the first floor. Here is the first floor plan.

First Floor

That extender works well if I connect one device(Phone) with it . But whenever connect another phone , signal drops. For example, In bedroom 1, I get around -48db~-52db signal strength on my phone . But If I connect another phone, then signal goes down to around -62db~-67db. Situation get even worse if i connect another phones, Signal goes down to -72db or higher. Youtube Video keeps buffering, Can't browsing smoothly.
For testing, I connected the phones with my router from ground floor. i am not having any issue there.
Is it normal? or I got a bad product..
I bought it from Amazon, and it comes with 10 day replacement/refund warranty.
What should i do ?

** One thing, We use wifi only for phones and other devices (Computer, Xbox.etc) are connected via Ethernet .

Please help.