Mapped network drives are not available in RDP sessions even though you have selected All drives; or are not displayed in the dialog when attempting to add Specific Drives using the More button.
RDP Local Devices and Resources details
This problem happens when you start Remote Desktop Manager with elevated privileges (Run as administrator). Mapped drives are not available from an elevated prompt when UAC is configured to "Prompt for credentials" in Windows
This issue is not caused by Remote Desktop Manager, you will need to apply a fix on all workstations from which you must launch Remote Desktop Manager with elevated privileges.
Using the Local Group Policy Editor, locate the following Group Policy path:
Local Computer Policy\Windows Settings\Security Settings\Local Policies\Security Options
Configure the following policy to Prompt for consent:
User Account Control: Behaviour of the elevation prompt for administrators in Admin Approval Mode
Map the required drives again in the elevated session. A good solution would be to create a batch/command file that starts by mapping the drives again, then launches Remote Desktop Manager.