If the computers are not communicating with SEPM, that may lead to having the computer name and other details be blank.
On the client workstation, please check if there is a green dot on the shield icon which indicates the successful communication.
If you have green dots on the clients, then you can open SEPM->Clients tab, right click on theclient you are trying to convert and go to properties.
Under the General tab, you should have the computer name listed.
If you find that the computer is communicating with SEPM, but no information is being shown in SEPM, then you an delete that client from SEPM. On the workstation, right click on the shield icon and click on update policy. That would force a re-registration of the client with SEPM that will include the transmission of client information.
Cheers,
Aniket