Where are you not seeing multiplayers ?, on the world map or once you’ve started a flight or both ?
See report about not seeing multiplayers on the world map: World map - multiplayer aircraft not showing
See also Multiplayer doesn't work. No players, no nameplates, nothing. No matter the server or location
In which version are you ?, Live version 1.4.20.0 or the SU3 Beta 1.5.9.0 ?