Drive Unlocker doesn't mount disk

I have 20 or so external drives for backups, from 1TB to 8TB. They are password protected. ‘Suddenly’ the password unlocker won’t mount the disk after entry of correct password. And yet if I try running it again it says the drive is unlocked. But the drive is not there. I have to Eject the Unlocker ‘cd’, run down the drive and start again. It can take from 3-8 tries before it suddenly announces my drive is available.

Any ideas why this has ‘suddenly’ started? It happens on multiple disks, different sizes. I’m using a PC running completely updated Windows 10.