Device Control Blocks Allowed USB drive, but log says that device was allowed

Good Morning,

We have set up device control rules using Symantec Endpoint Protection Manager 12.1.5. The AV Client version is 12.1.5337.5000.

We block all USB drives except a few whitelisted ones. I have recently added a few new drives to the list and encountered unexpected behaviour. When the user plugs in the drive, they get a message saying that the drive has been blocked, but when I look at the device control log, the log states that the device was allowed. The drive does not show up in Windows Explorer.

How can I get the USB drive to be allowed on the client computer(s)?

Thanks,