Windows 7 Bootcamp mouse & keyboard not working
I'm installing Windows 7 Professional via bootcamp on about twenty late-2012 Mac minis, all with OS Yosemite 10.10.1. Some of them work fine - I'd say about 75% of them so far are installing correctly, but the others will install Windows 7 and the bootcamp drivers, update to the latest version of Windows, restart and... no more working mouse/keyboard, regardless of how many times it's restarted or unplugged/plugged back into the USB port.
Both the mouse and keyboard will work in Safe Mode, and OS X has no problem with either the keyboard or mouse after it asks 'which key is next to the shift key'. I'm completely at a loss, since over half the computers I install Windows 7 on work just fine, while certain computers refuse to recognize the mouse/keyboard. (I've actually installed and reinstalled Windows multiple times on the trouble machines with no success.)
They're all using the same equipment, and all booting from the same USB drive when installing Windows. If someone could point me in the right direction, I'd really be grateful. It's got me pulling out my hair, here.
Mac mini, OS X Yosemite (10.10.1)