airplay multiple speakers not working in itunes

Hello guys.

When I try to stream music from iTunes Multiple Speakers on my iMac to the Airport Express it only plays 2-3 seconds of any song. The stream then is ended and only the iMac speakers sounds.

The same thing happens when I try to select any Apple TV. It plays 2-3 secs of a song and stops.

If I go to Preferences->Sound and I choose the Airport Express or any Apple TV as the output everything is all right.

Now, you can realize that I have 3 airplay devices and I can't use them all together. :`(

any ideas!??!?!?