Wired Keyboard on 2014 iMac not working

Hi there,

I have a lab containing 25 x 27" 2014 imacs. During a morning start up roughly 6 out of the 25 (and usually different ones each time) will be unable to detect the wired apple keyboard that is attached. All USB ports on every machine are working correctly. To resolve this issue, sometimes it works by just swapping the ports, sometimes a full reboot will do it, other times it comes down to swapping the keyboard with its neighbouring iMac (even if both are not detected) and miraculously they both are now detected.

Resetting SMC and PRAM does not fix the issue.

What is going on? All these keyboards work fine when attached to a 2011 iMac running the same OS (Mavericks.)

Thanks,

Dudley