hall two way gang switch

[garynic]

hi i have installed a down stairs double switch and a upstairs single switch. the switches turn my upstairs light on and off from the downstairs double switch or the upstairs single switch but if i turn on the upstairs switch it will not turn off from the downstairs switch or the other way around.my hall cable is connected red in com. black in l1.upstairs cable is red in com. black in l2 .with a red link connecting both coms.upstairs switch is one single red wire in com (black wire from cable not used) other cable i have red in l1 and black in l2 any suggestions please