When scanning for players, Manager Express broadcasts network packets to all devices on LAN.
If a player receives a packet from Manager Express, it will respond by sending its IP Address and Player Name to Manager Express.
However, in some places the network router does NOT allow broadcast due to security concerns. That is why scanning doesn't work.
If broadcast is not allowed in the place of deployment, please manually add player's IP to Manager Express.
Please also see:
How to manually add player IP address to Manager Express for exporting?