This sounds like you are connecting with Players that are not in the same circle of sharing. Is this from an invite game or a Random game? (auto-pick)
If you are inviting players, and they are accepting your invite to play, then there may be an issue. Otherwise, it is normal Google+ behavior to not give out the icon or name of Participants if circle matching isn't complete.
From Android Developer Participant
public abstract Uri getHiResImageUri ()
Returns the URI of the hi-res image to display for this participant. If the identity of the player is unknown, this will be null. It may also be null if the player simply has no image.
To retrieve the Image from the Uri, use ImageManager.
Returns
The URI of the hi-res image to display for this participant.
public abstract Uri getIconImageUri ()
Returns the URI of the icon-sized image to display for this participant. If the identity of the player is unknown, this will be the automatch avatar icon image for the player. It may also be null if the player simply has no image.
To retrieve the Image from the Uri, use ImageManager.
Returns
The URI of the icon image to display for this participant.
public abstract String getParticipantId ()
Returns the ID of this participant. Note that this is only valid for use in the current multiplayer room: a participant will not have the same ID across multiple rooms.
Returns
The ID of this participant.
public abstract Player getPlayer ()
Returns the Player that this participant represents. Note that this may be null if the identity of the player is unknown. This occurs in automatching scenarios where some players are not permitted to see the real identity of others.
Returns The Player corresponding to this participant.