Other participants are not visible when joining a conference room

On OpenFire 3.3.3 with Pandion 2.5.

For some users, other participants are not visible when joining a conference room though the invisible ones are active in the room.

Those who cannot see anybody in the room are connected from Pandion on a same terminal server, but the invisible ones are connected from a client PC.

Is there any idea to resolve this symptom ?