If the computer is in the unblock group and is showing in op manager then you will have to give access via local group policy
To do this:
Computer Configuration -> Administrative Templates -> System -> Removable Storage Access
Then open Command Prompt as an administrator, type gpupdate /force
Make sure sure the ADselfservice agent is installed via IMDSP server.
Use Endpoint DLP
Go to configuration
Add workstation
Tick Removable Device Auditing and then click the plus to add the workstation
Go back and find the workstation to install the agent. Normally it will have the message Agent not installed
Click on Manage Agent and then install Agent
One the agent is installed (may take a while) use the referesh by the agent service and deriver service. If they are both green click on the refresh next to configuration sync details. If this goes all green then it has worked.