PSU Control - Won't turn printer off

I have installed the PSU Control plugin and have wired up a relay between my power cable and raspberry pi, but it can't turn the printer off. I can see that clicking the button in OctoPrint is turning the light on/off on the relay module which suggests it should be working, but the printer stays on regardless! Any suggestions as to what might be wrong? I have already taped over the 5v pin on the USB cable and have unplugged the cable completely to confirm that is not the issue.

Is there anything else to try out, or is it just a faulty relay?