2 x HTC One (m7), MHL only works on one of them?

I have 2 x HTC One (m7), i have always used my original to watch video on my tv with mhl via an adapter to hdmi. Bought a 2nd hand HTC One (m7) exactly the same as a spare, but testing it out the mhl connection doesnt work. The phone recognises the cable connection because it starts charging, but the normally automatic mhl connection doesnt happen. Faulty usb port perhaps? (One dodgy pin?)