Do the faulty miggys show the boot/insert disk screen? If they do, listen for the drive-click noise - I'm sure you know what to listen for
. If there's no drive click then it's likely the drive itself could be faulty.
If you don't even get the boot pic it could be something more sinister. If you've not already had the tops off it may be worth opening the computers up and checking for anything obvious on the mobo, such as moisture damage, bits of metal or dust shorting tracks, loose FDD ribbon or power header connection.
If it's been a good decade or so since they've been used it's worth giving them a bit of a clean up and service.
Hope this helps.