Back button lights up but not working

Hello,

Just today i bought new board with charger connector back button home button and task button...(i dont know actually how the last touch button is called)

I replaced the old one with the new one,so left button reacts to touch and wrking, home button is working too ,but the back button on the right doesnt respond to my touch of finger it only lights up when i touch the task button on the left.so its brand new why it desnt work its not damaged at all i looked it very carefully twice.