When performing RDS shadowing, by default, the user whose session is being shadowed has to consent to viewing and/or controlling his/her session. This behaviour can be changed with a group policy.

In your group policy (local or through domain), navigate to:

Computer Configuration\Policies\Administrative Templates\Windows Components\Remote Desktop Services\Remote Desktop Session Host\Connections

Find the policy called Set rules for remote control of Remote Desktop Services user sessions and configure it: