Check the sudoers entry for the user to determine if there is a run_as user option set instead of ALL.
User1 ALL=(Admin1) NOPASSWD: ALL
Here the run_as user has been set to Admin1
If a run_as user has been included in the sudoers entry, then you will need to use the '-u' flag with your sudo command.
E.g the following sudo command would need to be ran when logged on as User1 in the previous sudoers example:
sudo -u Admin1 <command>
Note that this is a sudoers configuration issue rather than a Privilege Manager for Sudo issue.